You may hear that one of your friends paid you with Popmoney. Is this one of those scams designed to steal your bank account information?
Popmoney is legitimate (but you should of course make sure the text or email message is also legitimate -- from Popmoney, that is). It's a way to send money electronically, and it's gaining popularity as more and more banks make it available to customers.
What happens when you need to pay a friend? Perhaps you've shared a meal, or you need to pitch in for a group activity. You can write a check, pay with cash, or calculate some portion of the bill that you'll pay with plastic. Another (perhaps easier) option is to have one person pay while everybody else settles up, and electronic payment services make that easier.
It's getting a little bit easier every day. PayPal has been around for a while, and Popmoney is another option.