Not really a type of modelling, but a career path. A freelance model will typically advertise her (or his) services on the Internet, often through one of the large, respected modelling sites.

Some online agencies are listed in the modelling links page. Try OMP and Musecube. OMP has the larger audience, Musecube presents itself more up-market, both enjoy an excellent reputation (but you still have to watch out who you meet there).

Work can be anything from portfolio work to artistic work, clothed or nude, pro photographers and amateurs, and increasingly for stock photography. This type of work falls into paid work and prints for time, and can be an excellent opportunity for models who don't make it with one of the large agencies or who want to pursue modelling as a part-time career.

Requirements are totally flexible and only depend on what the photographer has in mind.