Normally, what you put a
guitar in to carry it and
protect it. Comes in two forms, a soft case which bends but has some padding and offers some protection, and a hard case, which doesn't bend, and offers more protection. Some cases even have pockets or containers to hold
guitar picks, music books, or spare strings.
Also, useful for hiding things when you want to carry something "questionable" around. Kinda like a
violin case, except for things bigger than a small
machine gun. My friend brought some
katanas to
high school one day. I'm sure the administration would have had a field day if they found that out.
Other things you can hide in a guitar case:
Lethal Frozen Fish
Your
PS2.
Large amounts of unmarked, unconsecutive bills
dem bones
Your cat, or your dog
A guitar
Many many more...