This article describes types of crampons. For information on using crampons to climb ice and snow (and even rock!) see Crampon Usage.
There are several things to consider when choosing crampons. All mountaineering crampons have either 12-points (usually) or 10-points (sometimes), including two front-points. The small "in-step" crampons (not to be confused with "step-in" crampons!), that lack front-points, are for hikers who need to occasionally cross hard snow, and are not suitable for general mountaineering.
Hinged vs. Rigid
The first choice is hinged vs. rigid. A rigid crampon has no flex, and makes walking hard; these are relatively uncommon in mountaineering, but are sometimes used for ice climbing. All crampons shown below (except the monopoint crampon) are hinged. This is an easy choice: choose hinged for general mountaineering.
The second choice is binding style. There are various synonyms for each type (some names are brand-specific), but I will refer to them as step-in, hybrid, and strap-on. If you look at the Wikipedia crampon page, it refers to Crampon and Boot Grading, with grades of B0, ..., B3 and C1, ... C3. These are rarely used in the United States.
A step-in crampon (again, very different from an in-step crampon!) requires a boot with front- and rear- welts that will support a metal bar. Although it looks nothing like a ski binding, it serves a similar purpose; the boot and crampon become locked together very tightly, and it is quick to put on. Generally, there is a lever in the rear that will lock into place. If you paid $350 or more for your boot, then it probably has the necessary front- and rear- welts. See Mountaineering Boots for more.
A hybrid crampon is similar to step-in, but it doesn't require the front welt. Boots that are compatible will have a rear welt (and any step-in compatible boot is also compatible with a hybrid crampon), but don't have anything special in the front. These provide a slightly more secure fit than strap-on crampons, but are not as secure as step-in crampons. The only advantage over step-in crampons is that they fit more boots. If you paid $200 or more for your boot, then it might be hybrid compatible. In general, instead of thinking of a hybrid crampon as having the "best of both worlds", it might be more apt to think of it as having the "worst of both worlds."
A strap-on crampon will bind to any type of footwear, even lightweight hiking boots. However, for best performance and to minimize the risk of losing a crampon, you still want a stiff boot. A heavy, all-leather backpacking boot works fine for most Sierra mountaineering (though you may be cold in winter), but a hiking boot with any pure nylon patches will probably be too light-weight (and even colder). The club's crampons are all strap-on types. They are great for walking and low-angle, but can come loose with repeated front-pointing (especially if not laced and sized perfectly), and also have poor performance on hard ice, since they are more likely to wiggle than step-in crampons. For the average Sierra mountaineer, strap-on crampons are probably the best choice due to their versatility.
Front point design
There are two types of front-points: vertically oriented and horizontally oriented. The advantage of vertical orientation is increased vertical stiffness, good for front-pointing on hard ice. The advantage of horizontal orientation is increased horizontal surface area, good for snow. But, you can use vertical frontpoints on snow, and horizontal frontpoints on ice. Horizontal frontpoints are a bit more versatile, and are more useful for the Sierra mountaineer, so the choice here is easy also: choose horizontal. Most strap-on crampons have horizontal front-points, since they are not designed for ice climbing.
Vertical Frontpoints. Designed for ice climbing. This crampon (which is hinged), can be converted to a monopoint design.
Material: steel vs. aluminum
Another easy choice: choose steel. Aluminum crampons are much lighter, and are a good choice for summer hikers, skiers (there are also specialized ski crampons - see below), or hikers on the Pacific Coast Trail - basically anytime when there's a chance you might briefly need crampons. But if you know that you will need crampons, take a steel pair. They last longer and will be fine in all situations; you do not want to find yourself mixed climbing or front-point up ice with aluminum crampons.
Aluminum crampons are designed for pure snowy glacier travel (free of rocks and ice) and short-lifetime. These often have 10 points instead of 12 (to save weight), and almost always are hinged, with strap-on bindings and horizontally oriented frontpoints.
Steel crampons are heavier, but more rugged; the default choice for mountaineering. (Every crampon on this page, except for the example to the right and the 'ski crampon' below, is made of steel)
Anti-balling plates (aka anti-bott plates) are attachments to crampons designed to prevent snow from clumping to the bottom of the crampon; this phenomenon is referred to as "balling up," and is very dangerous because the snow can ball up enough so that it is the accumulated snow that touches the ground, rather than the points of the crampon. Hence traction is lost. The loss of traction is usually unpredictable, and therefore very dangerous.
The balling effect is most pronounced with warm snow, either due to warm spring temperatures or afternoon sun.
In the past two decades, manufacturers have started producing more effective anti-balling plates designed to limit this effect. These do not work perfectly, but definitely provide some help. The basic idea is that they act as a smooth surface (and probably hydrophobic) so snow is less likely to accumulate. Most are plastic, but some are also rubber (in my opinion, these seem heavier than the plastic ones). Around 2007, most flexible crampons are sold with the anti-balling plate included; you can also purchase them separately. Below are some typical anti-balling plates.
Most "automatic" or step-in crampons are compatible with ski boots. In fact, this combination can be very secure and convenient for travel in the Sierra. Ski mountaineers who find themselves in their skis most of the time would benefit from carrying a set of lightweight aluminum crampons, like the Camp XLC390. These take up very little room in a pack and are the lightest crampons on the market. Ski mountaineers who appreciate substantial summit efforts off of their skis would benefit from a good set of steel crampons.
Most ski touring binding companies also sell "ski crampons" to go under your feet while you are ascending on skins. These attach directly to the binding and are designed for travel on snow (and are thus made of aluminum). Be sure to order ski crampons that are wide enough to accommodate your skis.
That's it! Recommendations for first-time buyers: steel, hinged, strap-on, horizontally-oriented frontpoints. An example of this type is the Black Diamond Contact Strap crampons, which is the type the club loans out.
Crampons are sharp and so should be stored properly when not in use to prevent damage to you or your other gear. Options for storage include carefully strapping to the outside of a pack (points together) or buying a specially designed bag, such as this Black Diamond model. There are also special rubber crampon point protectors that REI sells for less than $10. Alternatively, you can construct your own storage device from an old pair of jeans! Just cut off a section of the leg thigh material and sew one end shut.