a knife can come in many shapes and sizes.
usually consist of a blade made of some kind of metal or stone, with a handle at the other end.
there are fixed blade knives. (meaning that they don't fold, they need a sheath to keep from getting cut)
and folding (the blade usually folds into the blade.)
one of the most widely known is the switch blade. (just a knife that has a spring loaded blade, no big deal)
and then there are knifes that have the blade the slides out of the handle. and some that have handles that fold onto the blade.
then there are piece of crap fantasise knives, dont waste your money.
not a weapon unless used as one, a knife is a tool, that doubles as a weapon.
then there is the term, to knife.
means to stab or cut someone, but not necessarily kill them.
very similar to shanking someone. (although shanks are better at a stab, usually favored in the hammer fist or oriental knife hold.)