What are the many freshwater substrate types?

There are tons of different types of substrates, but many break down into the following categories;

1) Gravel

2) Sand

3) Barebottom (so no substrate in a tank)

4) Pebbles/River rocks

It really depends on what species you have, since many do require very specific types of substrate such as sand, or mixed substrate such as sand with river rocks for example.