"Paying it back" is an obligation.
"Paying it forward" is to give because you want to. I am taking part in a "Pay It Forward" with Farmchick at It's A Small Town Life. Her blog is a lot of fun to read and her photos are so pretty! I am now paying it forward here. Here is how it works:
If you want to join in the fun, just leave a comment saying you'd like to be included. The first three people to comment will receive a gift from me within the year. In return, you will post a "Pay It Forward" post on your blog and send a gift to three people who agree to participate. You never know when the gift will arrive in the mail, but I will send it between now and next April. It will be fun to learn about people in order to send an appropriate gift. The gifts should be from your heart and shouldn't be expensive. Just something to brighten someone's day. I look forward to see who wants to play.