Good explanation above. Just to add a bit more:
A procedure turn is one that is made with either of the following:
1.) A turn first of 45 degrees left/right and then another turn of 180 degrees opposite in direction to the initial one. Or with:
2.) A turn first of 80 degrees left/right followed by one off 260 degrees in the opposite direction.
The base turn is made by first of all proceeding outbound on a certain track/bearing from a fix for a set time or distance (depending on the category of the airplane) and then by making a turn in order to position the aircraft on a final approach track/course.