Features are for new work. This might be something totally new, or a change to some existing functionality that has already been accepted as complete by the client. If you're asking for a change on a story that hasn't been accepted yet, it should be entered as a comment.
Bugs should be used if previously accepted work is not behaving as expected. If you find a bug in functionality that has already been Accepted, please create a new Bug story and put it in the Backlog. If you find a bug in a story you're testing, please Reject the story and add the bug as a comment. For all bugs, please include as much detail as you can about what you were doing at the time so that we can easily reproduce the problem.
Releases are for marking major releases. These are the only type of stories that can have due dates. Pivotal Tracker will show you if we're on track to hit a due date by coloring the release blue or red.
Chores are for either technical, behind-the-scenes work, or are reminders for information we need from the client.