Note: please forgive the crude diagrams and terse descriptions in this help file. It is only a first draft.
The left chooser contains a list of branches for a particular category of revisions in an archive. Generally speaking, one branch represents "the official sources" (conventionally named the "devo" branch) while other branches represent more advanced development that will eventially be merged into the official sources.
Clicking on a branch name presents you with a list of versions within that branch. For example, the "devo" branch might be divided into "revisions pertaining to version 1.0" and "revisions pertaining to version 1.1".
If you click on a particular version, the version list will move to the left chooser, and a list of specific revisions within that version will appear in the right window.
The version list also contains some alternative links. You can view the (automatically generated) ChangeLog file for a version, view the revisions by who created them (the "Creators" link), view the list of all files in all revisions in that version ("the file tags") link, or look at a detailed change history in the display area (the "details" link).
A word of warning about the "file tags" link: normally you don't really want to view files by tag name -- that's an obscure feature for advanced users. Instead, to view files by name, you should click on the "files" link for a particular revision in the detailed change history shown by clicking on the "details" link.