So a tribe is a product of Steemengine.com They typically are going to have their own frontend(website) and their own token that you’re eligible to receive when you use their designated tag or tags. Palnet, Neoxian, and CreativeCoin are all tribes.
A Community is a new feature currently only available on the beta.steemit site, but will eventually be integrated into Steempeak and ESteem that allows for better content discovery and allows users to trend as a team vs as an individual. There’s a lot more features coming to communities, but that’s where things are now.
(edit) Also, steemit I think allows for 8 tags. If you want the two additional tags you can post on steemit then edit your post on steempeak. In all honesty though, additional tags, other than tribe tags are really not going to provide you much benefit here on Steem. I think posting in a Community is a far superior way to increase your reach.