Locknuts are an important part of many machines, including bicycles, cars and airplanes. They are designed to keep themselves "locked" in place along a bolt, or to keep other nuts locked in place along a bolt, by creating friction in various ways. There are a variety of different locknuts, and each is good for different jobs.
A nyloc nut, or a nylon insert locknut, is a nut with a nylon ring on the inside of one end. The nylon ring is narrower than the diameter of the bolt, but can stretch around the bolt if torqued. The side without the nylon ring is threaded onto a bolt first. Once the nut is twisted onto the bolt to its desired tightness, the nylon works to keep the nut locked in two ways. First, it increases the friction between nut thread and bolt threads by cause the bottom faces of the bolt threads to press more tightly against the top faces of the nut threads. Second, the nylon itself is pressing on the bolt, creating more friction.
Since the nylon bounces back to its original form when removed from a bolt, it can be reused many times. However, these nuts stop functioning as locknuts at temperatures higher than 250 degrees.
Distorted Thread Lock Nut
In a distorted thread locknut, there are certain threads, either at one end or in the middle of the nut, that are distorted—that don't thread on to the bolt perfectly—on purpose. These distorted threads on the nut are forced to thread onto a bolt. The distorted threads and the thread on the bolt are forced so tightly together that there is a great amount of friction between them.
Distorted thread lock nuts usually cannot be reused, because they will have new threads cut into them after being used the first time, and will lose their locking power. However, unlike nyloc bolts, they are good in high temperatures up to 1,400 degrees.
Note that a distorted nut cannot be made of a metal that is stronger than the bolt metal or its distorted threads will destroy the threads on a the bolt rather than lock to it. For example, never use a steel distorted thread lock nut on an aluminum bolt.
Jam nuts are any nut that is jammed up against another nut in order to lock in place the nuts' position on a bolt. A good jam lock is achieved this way: A first nut is screwed onto a bolt a little past its desired location. Then, a second nut is screwed onto the bolt next to the first nut. The second nut is held still with a wrench, and then the first nut is backed up with another wrench to press tightly against the second nut—achieving a locked position on the bolt and also backing the first nut into its desired position. On the bicycle, these are used on hubs and bottom brackets to lock nuts just loose enough to allow rings of bearings to move freely, but not rattle around.
Jam nuts are used when the nuts need to be locked into place along a bolt without being pressed against another surface—they press on one another, instead. Jam nuts do not provide the an unbreakable lock, but they are sufficient, sometimes necessary, and they are reusable.
Split Beam Nut
A split beam nut is a nut with one end that is split into two or more segments (though the segments continue to be threaded). There is a gap between the segments, and each segment is bent slightly inward. The segmented portion of the nut threads on last, and as it threads on, the segments are forced to expand back out to fit the diameter of the bolt. However, they are still pressing inward, providing enough friction to lock the nut into place on the bolt.
Serrated Face Nut
Serrated face nuts have serrations cut into one or both of their faces. A face with the serration is screwed onto a bolt until it presses against surface, and the angle of the serrations cause the face to bite into the surface it presses against, preventing loosening. Serrated face lock nuts won't work if locked against a washer—the washer will spin instead of putting up resistance against the threads.