1:
The Document is basically where you start writing. After taking a minute to read the link in the document (which explains how long your document has until it's automatically deleted, this will only be an issue if your collab partner goes away for an extended period of time) you can delete the text and start writing.
2:
The Text Options are similar to Word, you've got (from left to right)
bold, italic, underline, strikethrough, bullet list, indent list, unindent list, clear Authorship Colours (see #5),
Undo, and
Redo. I'm certain you know how to use those.
3:
The Pad Options - you can toggle Authorship Colours (#5) and the line numbers (the little grey numbers on the Document) from here. Personally, I'd advise you to keep Authorship colours toggled ON, so you can see who wrote what.
Import/Export is where you can import a text document of your choosing into the pad - this is a good idea if someone goes on holiday for a week, if you copy the document onto Word and save it, you can import the whole document onto a new pad when they return.
Saved Revisions shows you each 'stage' that people have saved at, and you can load any one of them.
Time Slider does what it says on the tin - you can move backwards and forwards through the production of the document.
The last two options, Saved Revisions and Time Slider, aren't as well-used and you shouldn't -really- fiddle with them because you might end up loading an old version of the document and accidentally deleting someone's paragraph!
4:
Save Button. If you finish a paragraph, or finish typing your part of a collab, press it before you close the document to save your work.
5: I don't -really- know what to call this one, so I usually call it
The Roster because it shows you who's viewing the pad at the moment. Pick one of the 8 colours, give yourself a specific name (I'd go for your RPerguild username) and it'll pop up in the Roster. This is useful because it shows you which colour is which, and who's typing what.
6:
The Chat Window is a good place for OOC chatting. You notice the date in grey? Anything you type in the chat window is saved too, so you can leave messages to offline collab partners.
7:
The URL is key to any Titanpad user. By copying this short and unique URL and pasting it in a PM to your collab partner, they'll be able to access the same Pad and you can write together.
Bazmund