The DB structure is pretty self-explanatory. Some tables have a legacy "hd" prefix, like "hdUsers", newer table don't have it. Here's a quick overview:
hdUsers - stores user accounts hdIssues - stores support tickets hdComments - stores replies (AKA ticket-comments) hdFileAttachments - has file attachments for both tickets and knowledge-base articles hdCategories and hdSections contain ticket categories and sections respectively hdCommentTemplates - has canned responses KBArticles - for knowledge-base articles hdCustomFields - custom fields for both tickets and assets hdAssets - assets hdCategoryPermissions - store user permissions to particular categories (i.e. is he a "technician" for a category) etc. etc.
Most table names are pretty self-explanatory so you'll have no problem identifying which table has which data.