Performed on a group of processes, identified by a communicator
Substitute for a sequence of point-to-point calls
Communications are locally blocking
Synchronization is not guaranteed (implementation dependent)
Some routines use a root process to originate or receive all data
Data amounts must exactly match
Many variations to basic categories
No message tags are needed
From http://www.pdc.kth.se/training/Talks/MPI/Collective.I/less.html#characteristics