Tow hooks are very simple tools -- they're just hooks that are bolted to the frame or mounted onto the receiver of a vehicle. The frame or the receiver is typically the only spot on a vehicle on which it's OK to attach tow hooks. Other places like bumpers or wheel axles simply aren't built to support the force of another vehicle pulling on them. The U.S. Department of Transportation (DOT) requires that every American-made vehicle has four attachment points in their frames, and it's through these holes driver can bolt their hooks. If for whatever reason you don't want to use these holes, you can also weld tow hooks onto the frame if you have the skills to do so.
Once the hooks are mounted onto the frame, an appropriate set of straps is necessary for making a connection and allowing one vehicle to pull the other. There are actually two types of straps used in towing -- tow straps and recovery straps -- and it's important to know the difference between them. Tow straps are designed for, as the name suggests, towing. The straps come with a hook on each end, and they won't stretch. If you're pulling someone out of a rough spot while off-roading, these types of straps are not recommended because of safety issues -- more on that in the next section.
That's what sets them apart from recovery straps, which are the type of straps recommended for pulling out a vehicle that's stuck in the mud. Recovery straps have loops instead of hooks, and they're designed to stretch when pulling another vehicle. They slip right over the towing hooks securely, and when the vehicle doing the pulling starts to move, the recovery strap will stretch. Just like a rubber band or a Slinky, the strap wants to return to its original position when it stretches, so its energy is transferred to the immobile vehicle, pulling it out.
Using towing hooks and recovery straps seems pretty straightforward, but there are a few safety considerations to keep in mind that will protect both you and your vehicles from damage.