The Team Explorer add-in
We installed the
Team System release candidate yesterday here at Vertigo. It's primarily a server change, but all the clients do have to update their Team Explorer add-in.
What's the Team Explorer add-in? It
provides the core functionality of Team System:
This is otherwise known as
Team Foundation. The other Team Editions (Developer, Architect, Test) offer additional functionality
on top of the foundation. However, no Team Edition is required;
Team Explorer is a standard Visual Studio 2005 add-in, so it even works with Visual Studio 2005 Professional or Standard.
How do you know you have Team Explorer installed?
Check your about dialog:
Team Explorer also
shows up as a seperate item in add/remove programs, so it can be installed and uninstalled independently of whatever version of Visual Studio you may have:

The Team Explorer install can be kicked off from the
Team Foundation Server CD:

The Team Editions are functional without Team Explorer installed, but they're missing a giant chunk of functionality.
The Team Explorer install is the heart of Team System.