Many Git commands take revision parameters as arguments. Depending on the command, they denote a specific commit or, for commands which walk the revision graph (such as git-log(1)), all commits which can be reached from that commit. They are usually denoted as <commit>, or <rev>, or <revision> in the syntax description.
The reference documentation for Git revisions syntax is the gitrevisions(7) manpage.
Still missing from this page:
git describe, e.g. v1.7.4.2-679-g3bee7fb@ alone as a shortcut for HEAD@{-<n>}, e.g. @{-1}, and - meaning @{-1}<branchname>@{push}<rev>^@, for all parents of <rev>Needs separate documentation:
<rev>:<path> and :<n>:<path> syntaxA..B, A...B, B ^A, A^1, and revision limiting like -<n>, --since