Hairstyles are modeled by procedurally defined real functions with the use of “solid noise”, sweep-like technique, offsetting and set-theoretic operations, and non-linear deformations.

Hairy Cube (500K), more hairy pictures

More details on hair modeling can be found in the paper:

Sourin A.I., Pasko A.A., Savchenko V.V. “Using real functions with application to hair modelling”, Computers and Graphics , vol.20, No.1, Pergamon, 1996, pp.11-19.
Electronic version: PostScript (225K) and PostScript + gzip (49K)