At long last, the official MS marketing site for VSTS seems to not suck. As the definitive source I'll link it up front: http://www.microsoft.com/visualstudio/en-us/products/teamsystem/default.mspx
Still, let's summarize the 60 second elevator pitch. You've got four "role SKUs" that are each a superset of VS Professional + some additional set of features. You've got Team Suite, which is a superset of everything (like "Ultimate" editions in Windows/Office). And you've got Team Foundation Server which glues everything together with source control, defect tracking, build automation, project management, and deep reporting.
Role SKUs:
- Team Developer - profiler, static code analysis, code coverage tools
- Team Database - a full offline DB development environment, including DB designers, unit test tools, etc. Lets you program declaratively and let the Schema Compare engine do the dirty work. Hard to explain in a sentence -- go watch the Channel9 videos.
- Team Test - capture, tweak, and playback http requests as automated unit tests. Load & scale testing tools. Test list management.
- Team Architecture - SOA modeling tools. They roundtrip through the code editors by generating stub projects and validating final code on the backend to ensure it fits the defined architecture.
Note - Team Dev and Team DB have merged from a licensing POV. Buy one, get the other free.
All Team Editions get you an MSDN Premium subscription and a client access license for Team Foundation Server. Team Suite, as mentioned, gets you the kitchen sink.
TFS itself is a separate purchase, not available for download. (except in the form of a 180-day trial, or the 5-user-limited "workgroup edition" that comes with most MSDN subs) The TFS client is a Visual Studio plugin you can install into any edition of VS higher than Express, although using it with a non-Team edition requires purchasing a CAL.
VSTS also has some auxiliary downloads like the Build Agent (for spreading your build automation across multiple machines) and the Load Test Runner (does what you'd expect). See the licensing whitepaper for complete details on them.
Anyway, if all you want to do is set up a version control system and start coding, it doesn't really matter. The only TFS editions you'll find for download have hard limits, so choose your poison. And they will plug into pretty much any edition of VS once you install the client plugin (dubbed "Team Explorer"). Actually installing the server is not trivial, but that's another show....