Andrew Karpie's Online Work Arrangements Platforms presentation does a great job of explaining the changing ways contingent workers are sourced, engaged and managed by hiring organizations.
It also describes the rapidly growing and diverse online talent marketplace industry. The key slide in this description is below, but going through the entire presentation is well worth your time.
Online work arrangement platform (OWAP) is Andrew's fancy term for online talent marketplaces like Elance-oDesk, Freelancer.com, etc. He uses this term because the process of hiring contingent workers is much more complex than just finding or selling talent. He explains all this in the presentation.