So far, only binary trees have been discussed. However, in many cases, b-trees are just not enough. Why is that the case? As discussed in this chapter, b-trees are basically based on sorting. Operators <, <=, =, >=, and > can be handled using b-trees. The trouble is, not all.
Data types can be sorted in a useful way. Just imagine a polygon. How would you sort these objects in a useful way? Sure, you can sort by the area covered, its length or so, but doing this won't allow you to actually find them using a geometric search.
The solution to the problem is to provide more than just one index type. Each index will serve a special purpose and do exactly what is needed. The following index types are available (as of PostgreSQL 10.0):
test=# SELECT * FROM pg_am;
amname | amhandler | amtype
---------+-------------+--------
btree...