It means Professional Sports Authenticator. Along with Beckett Grading Systems, they're the leaders in grading cards.

You need to be a member to have cards graded directly, which means a membership fee and another fee for each graded card. Alternatively, you can find a dealer who is a member and who will submit them for you for a set fee.

Here's a list of their services, and a list of dealers with whom they work.

http://www.psacard.com/submission_center...

I'd also suggest reading more about Beckett, who have really become the top service over the past few years.

http://www.beckett.com/grading/