From the docs:
category — Gives additional
information about the action to
execute. For example,
CATEGORY_LAUNCHER means it should
appear in the Launcher as a top-level
application, while
CATEGORY_ALTERNATIVE means it should
be included in a list of alternative
actions the user can perform on a
piece of data.
MAIN means that this activity is the entry point of the application, i.e. when you launch the application, this activity is created.