The document discusses different types of indexes that can be used to organize data files on external storage. It compares file organizations like heap files, sorted files, and various indexing techniques including B-tree and hash indexes. It outlines the basic structure of indexes like B-trees, including leaf pages containing data entries and non-leaf pages containing index entries. The document also discusses concepts like clustered vs unclustered indexes, primary vs secondary indexes, and different alternatives for storing data entries in indexes.