Do not confuse the Go field with search. You can search each TWiki web. Enter a search string in the WebHome topic or the WebSearch topic accessible from the Search link on each topic. Go to the TWikiRegistration page to create your TWiki account. Your WikiName will be in this list after you register.
Topics are under revision control.
Diffs shows you the complete change history of the topic, e.g. who changed what and when. This web is the sandbox where you can make changes at will. Press the Edit link. You are now in edit mode and you can see the source of the page. Preview and save the topic.