WordPress is a great CMS, but it does lack in the user permissions department. By Default you can create a subscriber, someone who can do little more than view the site. A Contributor, someone who can create blog posts, but not publish them. An Author can create blog posts and publish them. The Editor can create and edit both posts and pages, and the Administrator who can do anything. But what if you have a user who you want to edit one page but not another? Its not possible with the default WordPress, but thats where plugins come in. The best plugin I’ve found for giving individual users and groups of users permissions is called Role Scoper. It allows you to set an individuals edition permission down to the most finite detail. You can also restrict access to parts of your site with this plugin, letting it do double duty. The only drawback about this plugin is that its menu system can be a little complex and confusing. It takes a while to know where everything is and what it does, but once you do, you can bend WordPress permissions to your will.
** UPDATE **
Role Scoper is no longer being update. The author has come out with a new plugin called press permit. There is a free and a paid version. The free version does not do all the things that Role Scoper did, but the paid version works quite nicely.