What is Gratitude?

We all know what gratitude is, right? We have manners - we thank people for gifts, holding the door open, nice gestures, etc. But what is gratitude really about? Does anyone know? What is the feeling of gratitude? Does it have a feeling? Does it invoke feelings? How does gratitude affect us in our every day life? With so many unanswered questions, I set out on a quest to find the answers about what gratitude really is and how it really works in our lives.