General terminology.
site
A site hosts trackers (optionally in categories). A site is the root of everything.
tracker
A tracker contains bugs. A tracker is the first level in the tree. A tracker has a description, and is optionally part of a category.
bug
A bug is an item in a tracker. A bug has a subject, a description, a status, a type. A bug contains emails.
The maximum lenth of the subject is 1024 bytes. The maximum length of the description is 65535 bytes. Status is one of: OPEN, CLOSED, UNCONF (unconfirmed), REJECT (rejected/not planned), UPSTRM (upstream issue). Type is one of: BUG, DISCUS (discussion), PATCH (automatically given to git patches)
An email is... well... an email. Nothing more to specify here, you know what an email is I hope.