There's a bewildering array of mystery goops available for use on threaded fasteners and fittings. Which one's right for the job?
Purpose: Lubrication while tightening, prevents fastener from seizing due to rust.
Use for: Any steel fastener that might seize up over time, including most things on engines (and just about everything that goes under a car or trailer). Brush a thin layer onto the male threads before assembly.
Removal: As easy as tightening it in the first place, even after years of being blasted with wet salt.
Purpose: Insulating barrier to reduce galvanic corrosion between mixed metals.
Use for: Anything threaded into aluminum. Coat the male threads completely with it.
Removal: Easy, the stuff's slippery as Teflon (since that's basically what's in it).
Purpose: Corrosion protection for electrified mixed-metal connections.
Use for: All connections involving aluminum conductors, or any mixed-metal wiring connection. (Not that you'd intentionally use aluminum wiring, but....) Work a nice glob of it into the mixed-metal junction before tightening it up.
Removal: No problem, it never really dries and is about as strong as grease.
Purpose: Helps a tapered thread make a fluid-tight seal.
Use for: Tapered pipe thread (NPT, etc.) only, never for straight threads or for fittings with O-rings. Commonly used in water, fuel and hydraulic piping. Put two to four tight clockwise wraps of the tape around the male threads before assembly.
Removal: It'll remain on the fitting when you undo it, and should be cleaned off before re-using the part.
Loctite Blue (242)
Purpose: Prevents fasteners from loosening due to vibrations and normal movement. Also see the slightly weaker purple version for very small finicky screws.
Use for: Electronic gadget housings, engine accessory mounting bolts, anything that might vibrate loose but that still requires easy removal. Put a small dab on the side of either the male or the female threads before inserting the fastener.
Removal: Easy to break free by hand as you loosen the fastener.
Loctite Red (271)
Purpose: Like Loctite Blue, but stronger and more resistant to intense vibrations.
Use for: Anything that'd really ruin your day if it were to vibrate loose, and that doesn't have to come apart for regular maintenance. Boarding ladder hinge bolts, steering gear bolts, rudder pintle nuts, etc. would be candidates. Use a small dab on either set of threads, it'll flow around them quite easily.
Removal: Elbow grease, a big wrench, and a torch that'll get the bolt over 260°C (500°F).
Loctite Green (290)
Purpose: Lock already-assembled threads
Use for: Things you need to seize up really well, but can't disassemble to coat the threads.
Removal: Heat and plenty of leverage, similar to Red.