At the Heart of Mechanical: The Key Switch
First and foremost, the thing that defines a mechanical keyboard is the key switch it uses. Most budget keyboards today use dome-switch technology, which registers a keypress when you type and push down a silicone dome and connect two circuit-board traces. (This technology is also sometimes referred to as "membrane switch" or "rubber dome," with minor variations in the essential design.) Though this style is easy and inexpensive to manufacture, pressing the keys requires a relatively large amount of force, which can result in a heavy, mushy feel to the fingers and a lack of either tactile or auditory feedback when you type. Plus, after a fairly "short" time (five million keystrokes, give or take), the domes can lose their springiness and either work less well or stop working altogether. So you'll probably have to replace the keyboard at least once or twice over the life of the computer that you use it with.
Mechanical switches, by contrast, get rid of the silicone altogether. Pressing down on the key activates a real, physical switch, usually involving a spring as the pushback mechanism, that registers what you type. Because the parts used are much more substantial than those in dome-switch keyboards, mechanical keyboards typically have a much longer life span. (Many boast ratings of 50 million keystrokes or more per switch, and may well outlast the first—or fifth!—computer you use them with.) The typing feedback also creates a more direct relationship between your fingers and what appears on the screen. Because of the hardware involved, mechanical keyboards tend to be thicker, heavier, and more expensive than their dome-switch counterparts. They are more of an investment, but one that will pay off in sheer satisfaction if the quality of typing really matters to you.
When shopping for a mechanical keyboard, you will want to pay attention, above all else, to the kind of switch it uses, and whether it offers auditory feedback (in other words, a click you can hear) or tactile feedback (a "bump" you can feel), or both. Also important is the amount of pressure that the switches require to activate (the "actuation force"). That will greatly affect its functionality and the possibility of finger fatigue.
The Classics: Cherry MX Switches
The best known and most frequently encountered mechanical key switches come from a company called Cherry Industrial. These "Cherry MX" switches come in a range of styles that offer different operation and feedback to better match with your own personal preference, and the work or play you plan to do most on them. (Note that most have an actuation point of 2mm.)
The different types of Cherry MX keys are named for colors. This rundown of the most common Cherry switches will help you better match what you need with the mechanical keyboards you can buy. Keep in mind that some keyboard makers use switches of a similar style, made by companies otherthan Cherry. But almost every manufacturer maintains the same basic "color" scheme and related traits to help keep confusion down. (So, for example, Cherry MX Blue switches, and Blue-"style" switches from other makers, both tend to be clicky.)
Cherry MX Blue
A close approximation of the old-school buckling-spring switch (see below), but with a new-style mechanism, Cherry MX Blue switches are both tactile and clicky. With Blue switches, you feel as well as hear the completion of a keystroke (via a bump when it activates, and a distinct click). These switches are ideal for serious typists (many of whom insist that the switches deliver a turbocharging bounce you can't get anywhere else), but they are not best for gaming applications, as they have a rather higher actuation force (50 centi-Newtons, or cN) than you might prefer for a fast-twitch gun battle.
Another potential downside of the Blues: Some people find the keys' audible click quite loud (and possibly, annoying), which may cause problems in close quarters, whether at the office or at home. An office full of Cherry MX Blue keyboards will sound suspiciously like a big-city newsroom, circa 1935.
Cherry MX Black
With the highest actuation force of the standard Cherry varieties (60cN), the Cherry MX Black switch can come across as stiff. This type is thus less suitable for the kind of nimble key work most speed and touch typists depend on, and fast-fingered gamers tend to shun it. But this makes Black an excellent switch for cases where precision is paramount: entering mission-critical data (say, for an accountant or at a point-of-sale terminal) or for certain kinds of more deliberate gaming, as you will seldom have to worry about accidentally striking a key twice. Cherry MX Black switches are also neither tactile nor clicky.
Cherry MX Red
Similar to MX Black, Cherry MX Red switches lack both tactile and auditory feedback. But they have a lower actuation force (45cN), so they can be hit more quickly and more often, giving you the edge in any game demanding ultra-quick input. MX Red keyboards tend to be favored by gamers who play games that require fast-twitch actions. These same qualities, however, keep them from being a good choice if typing is your primary activity, as they make it easier to register more keystrokes than you intend or to trigger typos on a slightly stray stroke. Certain highly precise typists, though, will appreciate their light touch.
Cherry MX Brown
If you spend about as much time scribing emails and Word documents as you do mowing down charging zombies in first-person shooters, the Cherry MX Brown switch may be for you. Its 45cN actuation force is identical to what you get from the Red switch and, like it, the switch isn't clicky, but it gives you the same typing-boosting tactile bump you get from Blue. It's often cited as a good balance for gaming and typing between the clicky MX Blues and the "fast" MX Reds.
Cherry MX Speed Silver
Much like MX Reds, Cherry MX Speed Silvers demand the same 45cN actuation force, albeit with a shorter actuation point of just 1.2mm. (Reds have a 2mm actuation point.) The total travel distance is shorter too, at 3.4mm as opposed to the 4mm travel distance of the Cherry MX Reds. Having to press down less of a distance contributes to these switches' namesake trait: speed. As a result, the delay between pressing down a key and performing an action is kept to a minimum, making Speed Silvers a refreshed favorite for gamers.
Other Cherry MX Switches
The above switches are the kinds you're most likely to find in a keyboard you purchase today, but Cherry's rainbow does extend a bit further, to a few much less common types. Cherry Clear switches are tactile like Brown, but possess a higher actuation force; Green switches can be considered stiff Blues, both tactile and clicky; and White switches are quieter Greens. Several other types have specialized uses (such as just for space bars), but they will seldom be identified as such on any package or marketing material.
The Non-Cherry Brigade
A number of companies make switches that either mimic or try to improve on the Cherry MX switch functionality. Some gaming-keyboard switches, for example, have shorter actuation points to register your keypress action more quickly. Razer, for one, recently developed a hybrid "Mecha-Membrane" variety that uses mechanical means to activate a silicone dome switch. We've seen this used in the likes of the Ornata Chroma, as well as in the Cynosa Chroma and its underglow-laden twin, the Cynosa Chroma Pro. But we'd consider these spinoffs as opposed to true mechanicals. (Cooler Master has offered similar "hybrid" switches.)
Razer also offers true mechanical switches, known as Razer Green (tactile and clicky), Razer Orange (tactile and silent), and Razer Yellow (linear and silent). Here's where one vendor goes off the color rails: The Razer Greens are most similar to Cherry MX Blue switches, Razer Oranges are closest to Cherry MX Browns, while Razer Yellows are congruent to Cherry MX Reds. Razer key switches exhibit unique travel distances and actuation points, too: Greens and Oranges are 4mm deep and actuate at 1.9mm, and Yellows are 3.5mm deep and actuate at 1.2mm. You'll want to try these before you buy as they are a world of their own.
Logitech's mechanical keyboards, on the other hand, have become increasingly popular for their homebrew Romer-G switches. These come in both Tactile and Linear flavors and are rated for a whopping 70 million keystrokes. Romer-G switches are outfitted with an actuation distance between that of the Cherry MX Reds and Silvers, and they require the same 45cN force to actuate. Furthermore, Logitech now posits GX Blue switches as an alternative to Cherry MX Blues. You can find all of these options in variants of the Logitech G513 Carbon$129.99 at Logitech G.
None of these has become quite as popular or as widespread as the Cherry MX switches, though, so for the most part they're not worth discussing in depth. If you come across a keyboard brand using an unfamiliar switch type, try to determine both its actuation force (explained above) and its actuation point (at which depth of the keypress what you type is registered). Compare these values with those of the Cherry switches, and you should get an idea of what you're in for. Trying them in person is also recommended; we've tried imitation Blue, Brown, and other switches, and noted some subtle and not-so-subtle differences in feel from the Cherry standard. One major, common maker of Cherry-like key switches is Kaihua Electronics, better known by its subsidiary brand Kailh. Kailh switches are often used in lower-cost mechanical models, and indeed, if you yank a keycap off a late-model mechanical keyboard and don't see the word "Cherry" on the switch, "Kailh" is the next most likely branding you might see.
One of the most unusual switches you can find is, in fact, a quintessential mechanical example. The buckling-spring switch was used in the now-legendary IBM Model M keyboards that made such an impact in the 1980s—some of which are still in use today. It can still be found in keyboards from Unicomp, the company that acquired the manufacturing rights to it. (The Unicomp Ultra Classic definitely lives up to its name.) Buckling-spring keyboards use a genuine spring to activate the switch; when it buckles in the middle as you press it, it causes tactile and aural feedback (the latter from the spring hitting the wall of the switch enclosure). Keyboards using this style of switch are rare these days, but they're prized for their unparalleled typing capability and psychological satisfaction.