What is prayer?

Prayer seems to be something Christians do a lot of, and there are about as many ways to pray as there are Christians...but what actually is it? A theologian once said of Adam and Eve that whoever they were, they were the "first humanoids who prayed." (It was Robert Jenson, for those curious.) So let's take that as solid evidence that prayer is important not only to God, but to the entire human condition...in other words, we're probably built to pray...(I know, I know,...the Darwinists among us--myself included--struggle to make sense of phrases like "built to pray"...but truth is truth.)
So if we pray because that is what humans do, or are least created to be capable of doing prayer, it seems to me we should spend some time thinking about what it is, as well as what we do. (The books on prayer are legion--in fact, I'd be willing to bet that almost the first books made of any language are some kind of prayer book--even for those languages that don't have the concept of God--take that under consideration you Wittgensteinians!) Prayer is constitutive of the human condition for most Christians...therefore, it's as optional as drinking water, resting, or having sex...
So...do we pray as if God owes us something? Do we pray as if God forgot something (The great joke Tony Campolo tells of praying for Sister Bertha in the hospital, and God responding "That's where she is...I was wondering why I hadn't heard from her recently!") Do we pray because we are on the last string of the rope, and all other options are exhausted? What do we pray for? Why do we pray for it?
If the emergent church stuff speaks anything, it speaks a way of praying that is decidedly NOT about asking God for anything (even guidance), but rather praying as listening to see if God's about. Mother Teresa in an interview once was asked what she does when she prays. She responded, "I listen." The interviewer then asked, "Then what does God do?" "He listens too," she said. I love that story, as too often prayer is seen as an activity of doing rather than receiving, of trying to make something happen or be resolved rather than listening for the presence of God. To do in prayer is to listen, not to talk...
I've given up trying to say things during prayers...rather, I'm trying to encourage people to listen...just in case, at that very moment, God is speaking. If nothing else, it seems like good manners.