It is called a snare drum because it has wires going across the center of the drum, underneath, when the drum is struck, it uses the metal wires to create a "sizzle" or "rattle" sound. This gives it the distinct sound that a snare drum has. And some people will just call a snare drum a snare for short.

snare drum

Have the snare drum and drumsticks on then dance!

The snare head and snare system

You can get several different sizes of snare drum i think the standard one is 14"

I'm not sure

But you can get different szes of snare drum