Meagan, with all due respect, have you read the manual yet?
The answers to these questions are in there starting at Chapter 20 (Saving, Rendering and printing Projects) and are explained pretty well.
When you “save” (File – Save) a project, all you’re doing is saving the project file which is the instructions to what you did as far as editing, graphics, etc.
“Rendering” (File – Render As) is the creation of your project in the file format of your choice.
Vegas gives you a lot of options here and which one you choose is dependent on your needs.
Check Edward Troxel’s newsletters for a variety of instructions on different things in both Vegas and DVD Architect.