The Jossverse is an Alternate Universe of the Real World created by Joss Whedon. It is the continuum which Buffy the Vampire Slayer and Angel take place in. It is also, debatably, the continuum in which Firefly takes place, although that would be far in the future, and the vampires and occult elements of the BtVS side of the Jossverse appear to have been lost.
The term may also refer to other works by Joss Whedon, such as Dr. Horrible's Sing-Along Blog or Dollhouse, so it should probably be used sparingly.