Are you talking about viewing the differences interactively in the RTC client, or generating a list of changes?

To see the changes between two builds interactively:
- double-click the build definition in the Team Artifacts view to list the builds in the Builds view
- select two builds
- choose Compare Builds from the context menu

For the common case of seeing the changes that went into a given build (since the last one), you can just open the build result and click the Changes or Work Items links.

Note that this works by comparing snapshots for the builds, so it doesn't apply for personal builds, since they don't create a snapshot.

For generating a log of changes, this can be done using the SCM CLI, but it's pretty tedious. We're working on addressing that for 3.0; see 59822: ANT task to generate text file with change set descriptions.