Writing Plugin Help

I’m trying to find the best approach to writing a plugin that does the following:

Creates a new type of entry (as opposed to post, page, and link).
This entry will have 5 fields and will be assigned to one or more categories within wordpress.
These entries will be manageable by contributors.

Does this make much sense? How should I go able adding this into the current wordpress database? Thank you for any assistance! This is my first real database-driven plugin attempt