An album is a shareable object; you can add photos to an album, then subsequently share that album with someone else. It's better than sharing your whole library, and allows you to make and share lots of different related sets of photos.
Group Albums allow you to collaborate on an album with others. Both you and the collaborators can upload photos or videos, and view them in My Library. All members of the Group Album can also edit, copy, print, and download the items in the album.
Only members of Photoshop.com can participate in Group Albums. But since membership is free, you can invite nonmembers to participate in a Group Album, and they can create a Photoshop.com account and contribute files.