AAP: Association of American Publishers.
ALA: American Library Association. Also refers to their twice a year meetings where they vote on and honor recipients of the Caldecott, Newbery and other prestigious awards.
ARC/ARE/galley: Advance Reader Copies/Editions. These are the uncopyedited manuscripts in book form, sent out for early reviews and publicity. Read this excellent post by Literary Agent Holly Root on what ARCs are, and what they're not.
BEA: BookExpo America, an educational show.
CBC: Children’s Book Council.
Copy/flap copy: The blurbs on the flaps (or back cover, if it’s a paperback) with an excerpt from the book, review quotes, about the author, etc.
MG: Middle grade. Books for tweens.
PB: Picture book.
Slush Pile: A pile of manuscripts that agents, and occasionally publishing houses if they allow it, receive from unagented authors looking to publishing their manuscript.
Query Letter: A short, concise letter in which an unpublished author pitches their manuscript in order to get an agent. See QueryShark for how to write, or rather, how NOT to write a query letter.
YA: Young Adult. Books for teens.
YPG: Young to Publishing Group. An initiative of the AAP, for those in publishing 0-5 years.