Assigning Authors vs Contributors

Hi,

I am new to WP, and have a question regarding setting up new users for my blog. I would like to have a couple people who can publish articles, but would not be able to edit themes, sidebars, etc. Would I make them a Contributor?

And, is there any way I can limit where they can post? For example, if I have someone who contributes to articles in the Business category of my blog, can I limit them to that category only?

Thanks so much,
Vicki