Loctite is the original, and has become the generic term for thread-locking compounds the world over. It is one of the first anaerobic products, meaning it does its thing when deprived of air. Apply the product to clean parts before assembly, and it will harden into a plastic-like seal/adhesive. It comes in several varieties and is primarily used for locking threaded fasteners.
Loctite 240-series is used for medium-strength applications, such as nuts that you want to keep tight, but need to be able to remove with hand tools. 243 also seals fasteners that are exposed to fluid pressure, like brake adapters or water pumps.
The 270-series red is high–strength and is used to mount studs or bolts that you do NOT want to come loose. (In the event that you ever need to remove the fastener, it is possible to break the bond either by applying a large enough torque or by applying heat. At 500 degrees, the stuff gives up and looses its strength, turning into a crumbly powder.)
290 is a wicking grade, medium strength locker that you can apply to fasteners after assembly.
(For some idiotic reason, the last bottle of red loctite I bought was made from blue plastic, hence the rattle-can paint job. Easier to reach for the right stuff that way.)
Loctite 620 Cylindrical Retaining Compound is used to lock bearings into housings, or even a metal-backed oil seal into an engine's front cover. I use it to lock replacement bushings into carburetor bodies and to lock throttle levers onto the shafts.
Loctite PST Thread Sealant is a thick Teflon paste for sealing threads and pipes. (Do not use Teflon tape in automotive applications. The small shreds of tape can wreak havoc on fuel components.)
These are just a few of the common Loctite products. There are hundreds of them for use in everything from electronics to aerospace. Check out the Assembly Products Page.