The B'omarr Monks are the founders and soul members of the B'omarr religion. They had a monastary on Tatooine that became Jabba's Palace. They appear in Star Wars Return of the Jedi.
When C3PO and R2-D2 entered Jabba's Palace they saw an enlightened monk, the highest rank of the B'ommar (which is basicly a brain in a jar) walking through the halls on it's spider walker droid.
This is the only time they are seen in the movie itself, but their is more information on them later on. After Jabba dies, the B'omarr Monks banish all the straglers from the palace. But when Bib Fortuna refusses to leave and demands ownership of the monastary, the angry monks harvest his brain an turn him into one of them.
The B'omarr are a religious order who believe that cutting themselves of from civilization and worldly distractions will lead to enlightment. No one species makes up the B'omarr. When a monk reaches the highest level; their brains are harvested and placed in jars. This class has learned to communicate telepathicly. On the rare occasions that they do need to move around, they have themselves connected to spider-walker droids that allow them do wander the palace.
The B'omarr Monks of Tatooine allow crimelords to rule the upper levels of the monastary in return for insurance (as well as an additional chamber on the on the lower levels for them to use). As holy as they make themselves out to be, B'omarr Monks are not above fraternizing with their criminal guests. Whenever the crime lord running the palace is defeated, the monks drive out all stragglers and "convert" those who refuse to leave.