Here's a good article on wikipedia about the different sizes. I have also included a photo of both pages side by side, to illustrate the difference.
The one with the book on it is the US letter size. Having different sized paper feels significant, or am I blowing it up? I'd like to have the letter format for creative processes as it seems less portrait-kind. For letter writing and text documents I like A4 as the lines are shorter. I wonder what you think.