Any form of music that is made by nerds, for nerds, or about nerdly things. Nerdcore can be made in any style of music, but most people identify it in either its pop-punk or hip-hop forms. Although many people think the term "nerdcore" was made up by MC Frontalot, it has in fact been in use by rock bands for many years prior to Frontalot's use of the term.
Nerdcore should not be confused with bands that are simply stereotypically popular with nerds such as They Might Be Giants, DEVO, Ween, etc.
Musicians in this genre include (but are by no means limited to):
Rock:
Treephort
Nerf Herder
Warp 11
The Aquabats (definately moreso in their later albums)
The Phenomenauts (Mainly because of their pre-occupation with science)
The Mathematicians
MJ Hibbet
Hip-Hop:
MC Frontalot
MC Chris
MC Lars
MC Hawking
MC Paul Barman
Del Tha Funkee Homosapian (specifically on the "Deltron 3030" album)
Optimus Rhyme
The
Lords Of The Rhymes
Electronic:
The NESkimos
The Laziest Men On Mars
Industrial:
Kompressor
Parody:
Weird Al