First of all Team System comes in three different "flavours", Architect
edition, Delveloper edition, and Test Edition.

Architect edition includes all the componentes necessary to make system
diagrams, logical datacenter diagrams, SOA, classes diagrams, and validate
them, BUT does not come with developer tools creating any other kind of
projects  (class libraries, web sites, etc etc)

Developer edition comes with tools for developing (web sites, winforms,
class libraries, etc), that includes also class diagrams, unit testing (only
unit testing), code coverage tools and static code analisys tool.

Test Edition comes with all the necessary tools for testing, that includes
unit testing, web testing, load testing, code coverage, but as in architect
edition you can't develop web sites and so on.

And finally, answering to your question, Team Suite, comes with ALL of these
tools, I mean, it has all the tools from the three different versions, that's
why the price difference.