The trunk (boot, if you are talking about it in Europe) is a space within the vehicle which can be used to store items. The trunk lid is part of the body and is often where an optional wing or spoiler is mounted to the vehicle.
Many trunk lids are equipped with lift supports, so when they are opened, they remain open, allowing people to load or unload without having to hold the lid up, or prop it up with a metal rod which could get in the way or lost. Some contain the center brake light that would otherwise be mounted behind the rear window.
Most trunks in modern vehicles have an emergency handle on the inside, which would allow anyone trapped inside to open the lid. This latch is built to override the lock, and often has glow-in-the-dark plastic or paint on it.