Gamesharing
Some titles for the PSP support a feature dubbed "gamesharing," which facilitates a limited set of multiplayer features between two PSPs with only one copy of the game UMD. A reduced version of the game being shared is transferred to the PSP without a UMD via the PSP's Wi-Fi connection, whereupon it is loaded into RAM and runs. Such "gameshare" versions of titles usually have their feature set reduced because of technical limitations (small RAM size, slow bandwidth of 802.11b connection).
Read more about this topic: Play Station Portable Hardware