If you're considering purchasing one or the other, be sure to download the free 30-day trials from adobe.com
But, Dreamweaver is usually where web developers will code and lay out a website, order and construct the site's skeleton. Once the site is online, small changes are usually done through Adobe Contribute, especially when there are numerous people involved, like updating a blog, adding a new product to an online catalog, etc... (when people 'contribute' to the already built site)
That was my opinion of the differences, however other people will have different opinions.
Also, if you're thinking of buying one or the other to make a website from scratch, get Dreamweaver. Contribute is not meant to be making websites from scratch, it is limited in functions for that matter.