A bundle is a collection of macros, commands, snippets, drag commands, templates, preferences, and language grammars.

For more info see the manual entry about bundles.

Bundles are kept in the subversion repository. Here are instructions for anonymous subversion checkout.

If you have written a bundle that may have public interest you can write bundles at textmate.org to ask for it to be added to the shared repository (in the Review folder), but be sure to first have read the style guide.