I have 2 questions. What is a sprites Backdrop, and why would you need it, and Why would you need the User ID sensor block? If you can help me it would be much appreciated!

The “backdrops” are the 1.4 equivalent of backgrounds.

A backdrop is just what a background was in 1.x– a “costume” for the stage.

The user id block is less useful right now (maybe you could use it to count views in your project), but it will be more useful once cloud variables come back.You could use it to keep track of individual high scores or you could use it to make sure someone can only take your survey once, etc.

If you're still confused about user id's, read this http://beta.scratch.mit.edu/forums/topic/1326/