Longform

Many improvisers divide the craft into 'longform' and 'shortform'.  Longform is certainly harder to master and therefore see as a sort of holy grail by many. 

LONGFORM – An extended improvisational format.  Sometimes used by improv teams who find that the break between the short sketches they present gives audience members a chance to leave.

Jerry Schaeffer's Devils Dictionary for Improvisational Theatre