Grant Fritchey provides an overview:
In this article I’m going to go over the different types of indexes and some index behaviors. We’ll get into what the indexes are, how they work, and how best you can apply them within your databases. I’m hoping you’ll develop an understanding of which indexes are likely to work better in each situation.
However, I won’t be covering the indexes in detail. Further, I won’t be going very far into how these indexes affects performance. I’m not yet up on performance tuning within PostgreSQL. That article is quite a way down the track from here. When you know which index to use where, you can then drill down to better understand that index, and how best to measure the performance impacts.
This set of indexes is quite different from what we’re used to in SQL Server and much closer to what you get in Oracle.