book proposals and pitches

A pitch is used in fiction in an informal dialogue between an editor and writer OR writer and agent OR an agent and editor. It's a couple sentences (one is best) that encapsulate the book. For example mine for LOTTERY was Forrest Gump wins PowerBall.
Proposals generally are for non-fiction. In nonfiction typically you can get an agent and sell the book on a proposal, table of contents, and first few chapters. Your book is then tailored to how the publisher wants it.
For fiction - if you are unpublished - your book has to be completed. There are some exceptions to all of this of course. For example my second book will be sold on only a premise and first few chapters even though it's fiction as I already have an agent...