XKeyscore is the PRISM db search plugin/front end. There's a giant vacuum of everybody's personal data and XKeyscore allows you to see it in real time while it's archived into PRISM or look it up afterwards.
Bruce Schneier: First, be careful with names. PRISM is a specific NSA database, just a part of the overall NSA surveillance effort. The agency has been playing all sorts of games with names, dividing their efforts up and using many different code names in an attempt to disguise what they’re doing. It allows them to deny that a specific program is doing something, while conveniently omitting the fact that another program is doing the thing and the two programs are talking to each other. So I am less interested in what is in the specific PRISM database, and more what the NSA is doing overall with domestic surveillance.
http://blog.ted.com/2013/07/17/security-experts-on-the-nsas-...
Bruce Schneier: First, be careful with names. PRISM is a specific NSA database, just a part of the overall NSA surveillance effort. The agency has been playing all sorts of games with names, dividing their efforts up and using many different code names in an attempt to disguise what they’re doing. It allows them to deny that a specific program is doing something, while conveniently omitting the fact that another program is doing the thing and the two programs are talking to each other. So I am less interested in what is in the specific PRISM database, and more what the NSA is doing overall with domestic surveillance.