Even though the website http://office.microsoft.com provides a comparison of the different Office 365 plans, it is not always entirely clear which features are available or what the restrictions are.

Luckily, Jasper Osgaard created a great blog post comparing the different plans in more detail without having to go through tons of documentation:
http://blogs.technet.com/b/lystavlen/archive/2013/03/30/office-365-comparing-p-m-and-e-plans.aspx