Steam beta update?

I think that is the beta update that updates Skyrim to 1.4, and I think that's it. If you don't have Skyrim it's pointless to join that beta.

Steam pushes out beta updates to the client for users who opt-in to the beta; see here for details.

You probably won't notice any significant difference with the beta update. Just because its a beta test doesn't mean they are testing major or obvious changes, they also test "under the hood" stuff that you will never see... unless they break something in the client.