From Wikipedia:

"Geek Rock (also known as nerd rock, but distinguishable from the hip hop genre Nerdcore) is a musical subgenre of alternative rock, although unlike many genres, the term is somewhat loosely applied as far as the style of music performed is concerned. Rather, the phrase is more often used to describe the artists and performers, whose personalities and/or appearance are considered "geeky" or "nerdy".

However, there are certain common musical traits that many geek rock artists share, although a good number of the bands described as such may display a sound far removed from the considered standard. Such elements include heavy use of synthesisers and electronic keyboards, vocoders, harmonic vocals (and, sometimes, extensive use of female-led backing vocals) and idiosyncratic use of instruments not usually associated with alt-rock, such as accordions.

In addition, a number of lyrical themes are quite common to the genre, including themes of isolation, loneliness and failing love lives, and a fascination with "geek" pop culture such as video games, comic books, science fiction and fantasy. In addition, significant doses of irony and humor can be found in many geek rock lyrics.