ASCII "art" is any kind of image formed from symbols and pronunciation marks instead of being drawn by brushes or pencils. Ascii art was first made by geeks with too much time on their hands. This is like MS Paint on notepad. The first Ascii art was a crude attempt at porn so that the geeks could have something to masturbate to. An example would be this:
---,_,---- / . \ / | \ ( @@ ) / _/----\_ \ / '/ \` \ / / . \ \ / /| |\ \ / / | | \ \ / /`_/_ _\_'\ \ / '/ ( . )( . ) \ `\ <_ ' `--`___'`___'--' ` _> / ' @ @/ =\@ @ ` \ / / @@( , )@@ \ \ / / @@| o o|@@ \ \ ' / @@@@@@@@ \ `
Eventually Ascii art wasn't needed, as computer started to not suck so much ass, and real porn could be had on computers (although geeks are still known to make ascii pr0n). Then Ascii art, in all its suckyness passed on like a venereal disease onto forums, IRC and Facebook, where Ascii art is now used to spam peoples comment sections, and show how 1337 you are.