what is a `terminal`

i have a doubt, what is considered a `terminal` in the linux world? is it a keyboard, or a monitor? or maybe the combination of both?
also, and related to the question above, what is the information on the /usr/share/terminfo used for?
my terminfo directory is divided onto 4 directorires, each one of them filled with files with strange names like:

l/ lisa, luna, layer
v/ vt100, vapple, visual
u/ ultimaII, unixpc, uts30
x/ xterm, xenix, xerox, xtalk

the questions are, can i get rid of some of them? how do i know what i`m using, i want to delete everything else.
thanks in advance.

http://webopedia.internet.com/TERM/t/terminal.html

thanks a lot, dumb as it sounds, i didn`t knew what a terminal was.