Difference between "Pick a brick" and "Bricks and pieces"
The Pick-a-Brick selection has the more common pieces that aren't specific to sets whereas on Bricks and Pieces you can get set specific pieces that are more uncommon such as minifigure parts from a (non-licensed) theme. Since the Bricks and Pieces parts are more specialized they are usually more expensive than Pick-a-Brick, so don't get basic bricks from Bricks and Pieces.