Help Getting a NAS or Whatever They're Called

I don't know if I need a "true" NAS, whatever that is, but I would like to have some wireless storage ability. I've read a lot of review on Amazon, but every storage device seems to be either slow or not compatible with a certain media. I'd love some suggestions to pursue. I'd like to use the drive for time machine and as a storage device for a growing photo and video library. Some redundancy would be nice. I'd also like to be able to play iTunes off it. My network is set up with an Airport Express (802.11 N) that's connected to my router/modem. Thanks for any suggestions!