I didn't know this was going to be Loctite 101.
http://www.loctiteproducts.com/threadlockers.shtml for more info.
Loctite® Threadlocker Blue 242® is designed for the locking and sealing of threaded fasteners which require normal disassembly with standard hand tools. The product cures when confined in the absence of air between close fitting metal surfaces. It protects threads from rust and corrosion and prevents loosening from shock and vibration. Loctite® Threadlocker Blue 242® is particularly suited for applications on less active substrates such as stainless steel and plated surfaces, where disassembly is required for servicing.
Loctite® Threadlocker Green 290™ is designed for the locking and sealing of threaded fasteners. Due to it's low viscosity and capillary action, the product wicks between engaged threads and eliminates the need to disassemble prior to application. Loctite® Threadlocker Green 290™ cures when confined in the absence of air between close fitting metal surfaces. It prevents loosening from shock and vibration and leakage from shock and vibration and protects threads from rust and corrosion. The product can also be used to fill porosity in welds, casting and powder metal parts. Localized heating and hand tools are needed for disassembly.
Loctite® Threadlocker Red 271™ is designed for the
permanent locking and sealing of threaded fasteners. The product cures when confined in the absence of air between close fitting metal surfaces. It protects threads from rust and corrosion and prevents loosening from shock and vibration. It is
ONLY removable once cured by heating up parts to 500°F (260°C).
Don't take my word for it.