TM Bundle == ST Package (collection of plugins, snippets, options, language defs, etc)
TM Command == ST Plugin: python class derived from sublimeplugin.Plugin (or subclasses thereof). Interacts with ST's API and is one of the building blocks of a package (together with snippets, options, menus, etc).
Packge > (plugin, snippet, options, language def, menu file, build system...)
Only plugins ( == "commands" in TM) that are defined in the HTML package will be available to you (ok, to be exact: plugins defined for the text.html scope; they could be placed anywhere under your /Packages folder). The plugin could show up in a menu under Tools/Packages if it included a .sublime-menu file to that effect, but to my knowledge not many of them do at the moment. I you want to check out whether a certain plugin is available, you can look directly at the Packages/ folder. You can access this folder through the menu item Preferences/Browse packages...
That said, some users have created more sophisticated ways of "introspecting" your packages, if you will: look for the user "sublimator" on BitBucket.org, for example.
I reckon someone has done this already in a plugin... You could look at the Sublime Text repo on Google Code (easy to find on Google) or the aforementioned user in BitBucket.
Hope this helps! Anyone, correct me if I'm wrong!
EDIT: Oh, how I wish this board was sublimeoverflow.com...