The amount and type of data made available is virtually unlimited, though it may take some time to access massive remote data sets. I've prepared a small list of sample servers, selected to give an idea of the variety of information which is available. Or, if you're really lost, you can always quit, and start over.
Most graphical browsers have a menu button near the top left corner.
For example, it would be a real pain to find this document if you had to start at a top level server and wander around till you encountered it . A WWW browser should have some way to tell it specifically where to look for a document. If you move the mouse till the cursor is in the button, then single click, a new window should appear. Mosaic will now try to open the document that you named. If you mistyped something, or if the server is down or unavailable, it will try for a while, then tell you that it can't locate the document.