MVCC

MVCC is the method PostgreSQL uses to handle data consistency when multiple processes are accessing the same table. The manual documents it in the chapter on Concurrency Control.

There is a good example of how MVCC works in the PostgreSQL Through Pictures presentation, on pages 56-58, and a long discussion in the MVCC Unmasked presentation.

MVCC is not unique to PostgreSQL, it's a common database technique.