I'm on a study break at the moment, and I really feel like writing something inane about knives. So here you go:
Slip joints are the weakest of mechanisms; it's just a metal leafspring along the spine of the handle that holds the blade open. The blade is closed simply by applying enough force to overcome the tension in the leafspring. Most swiss army knives, folding whittlers, or any knives that can be closed with light pressure are slip joints. They aren't strong compared to other locking mechanisms, but they're legal pretty much everywhere.
Lockbacks (or synonymously, backlocks or spinelocks) are similar to slipjoints, and are one of the oldest locking mechanisms. Lockbacks use a leafspring, same as slipjoints do, but the leafspring has a protrusion on it which, when the knife is opened, fits into a slot in the blade tang. This locks the blade firmly in place, and when made precisely is quite sturdy. Instead of simply pushing down on the blade to close it, you must first press down on the base of the leafspring to raise it out of the recess in the tang.
The lockback is simple but has several drawbacks; any folder using it cannot be open at the back, which sometimes causes an inner buildup of dirt, lint and grime. Likewise any foreign matter that gets into the recess in the blade tang might interfere with the locking function, so regular maintenance is required to keep it working. Care must be exercised in taking a backlock apart, because the parts are under near-constant tension.
Examples: Spyderco Endura, Buck 110, Ron Lake Interframe, Gerber LST
The liner-lock was designed by Michael Walker about 30-40 years ago; when the knife is opened, part of the knife's liner snaps inward, fitting flush with the base of the tang and holding the knife open. To close it, the liner is pushed back outward with the thumb so that the blade once again has room to rotate shut. This locking mechanism is also quite sturdy, and many people find it to be more ergonomic than a spinelock, which usually requires two hands to close unless you're practiced or dextrous. On top of this, liner-lock knives can have an open build, allowing them to be easily cleaned or washed out, and even failing proper maintenance they are not nearly as susceptible to dirt as backlocks are. The drawback is that linerlocks can be more easily disengaged accidentally, when the knife suffers a hard impact.
Examples: Emmerson Commander, Cold Steel Ti-Lite, Columbia River Knife and Tool M16, Kershaw Blur
(Note: Knifemakers Michael Walker and Ron Lake collaberated on a system now featured in knives produced by Columbia River Knife and Tool, wherein a secondary locking mechanism snaps into place when the blade is opened and prevents the liner from accidentally disengaging.)
Framelocks are the same principle as linerlocks, only instead of a liner snapping flush with the tang, it's an entire section of the handle slab. The frame lock (synonymously integral lock) is functions in the same way as a linerlock, and has similar pros and cons, but is generally regarded as stronger because the area of the mating surfaces is so much greater. They don't fail as easily, though they are more likely to disengage if too much torque is applied to the handle - your hand might pull the lock out of place.
Examples: Chris Reeve Sebenza, Benchmade Skirmish, Kershaw Leek
There are a number of other, newer locking mechanisms as well, though they are a great deal less common.
Button locks are used often on smaller gentleman's folders; they have a variety of different mechanisms, and their durability varies widely depending on construction. To be fair, while they are used on one of the best tactical folders out there (MOD Dieter Mk. I & II) you don't often see them on tough working knives. Having never owned or used one, that's all I ought to say.
Examples: MOD Dieter Mk. I & II (as aforementioned), any William Henry knives
Sliding-pin locks have been gaining popularity in recent years. They go by a number of names, since each company that uses them has had to patent their own design, but in spite of the mechanical differences the function is essentially the same. When the knife rotates open, a spring-loaded pin slides into a slot in the blade tang, locking it quite securely. SOG has its Arc-Lock, the best in my opinion, Benchmade has its Axis lock, which is very similar but not as complex, and Cold Steel has the Ultra-Lock, which I've found to be functional but very finnicky. I hear Spyderco has something similar, as well. Most of these knives are very strong, and pretty slick to open and close, since the blade can be made to rotate freely when you slide the pin away. These knives are strong and slick, and can have open builds like a linerlock. The drawback is the same for button locks; they can be very hard to take apart and put back together for cleaning.
Examples: SOG Pentagon Elite, Benchmade Griptillian, Cold Steel Recon 1
The Stud Lock is one of Kershaw Knives' proprietary locks, designed by recent Cutlery Hall-of-Famer Ken Onion. The principle is like an externalized version of the sliding-pin lock, with the pin being mounted on the blade (in place of a thumb stud) instead of the handle. Since the pin can still be used as a thumb stud, and is in a very accessible place, the lock is very ergonomic. I used to have doubts about its strength, but I've owned one for awhile now and my apprehensions are disappearing.
Examples: Kershaw Spec Bump, Kershaw Speed Bump Kershaw Outcast
Most of the locks I've listed are reasonably strong, and if made properly have few fundamental flaws - especially those first four, who have stood the test of time. I've owned a number of knives with many of these locking mechanisms, and having worked landscape construction for several years, I've put them all through some hard use. I've never had a lockback or a framelock fail on me, though I have seen a few linerlocks come undone when I've dropped them or accidentally whacked them on something - luckily my fingers were out of the way. These days all my liner-locks are from Columbia River, and have the secondary safety mechanism to make me comfortable with using them. This is not to say they're bad; the lock is still used on some very reputable knives that I've yet to have the pleasure of using.
The important thing when buying a new knife is to get one that you are happy with ergonomically. Most knives with a triple-digit price tag will probably hold up to whatever use you put them through, as long as you don't actually wish to destroy them. It's really more important that you have something feeling good in your hand, and being there when you need it to be.
That's my (very lengthy) two cents. Back to studying.
