Once you have read and rated (on a scale of 1-5) a number of books, the site will start making suggestions for you based on the type of books you have read and the categories you have selected as some of your favorites. The site allows you see what others are reading, create widgets to put on other sites of what you are reading (see the widget to the right and the example on my About Me page.
Right now, one of my favorite parts about Goodreads is the First Reads program. Publishers and authors host giveaways for some of their books that you can sign up for at no cost. While a rating and review of the book is not required it is suggested as a thank you for getting the book for free. I was just notified this morning that I received the book Little Red Lies by Julie Johnston, can't wait for it to come in the mail.