Orbital element sets contain the data needed by software to predict the position of a satellite. There are two different data formats that are used: NASA/NORAD two line elements and the AMSAT format. All of the following sites distribute two line elements sets.
Most element sets come from NASA and/or NORAD and are distributed throughout the community. Most sites have the same or similiar element sets so you should get your elements from whatever site is most convienent and has the most recent ones.
Satellite element sets are posted to certain newsgroups and mailing lists.