The standard index type is the B-tree, where the B stands for balanced. A balanced tree is one where the amount of data on the left and right side of each split is kept even, so that the amount of levels you have to descend to reach any individual row is approximately equal.
The B-tree can be used to find a single value or to scan a range, searching for key values that are greater than, less than, and/or equal to some value. They also work fine on both numeric and text data. Recent versions of PostgreSQL (8.3 and later) can also use an index to find (or avoid) null values in a table.
