Midi to USB is basically a midi interface. That means there's some electronics in between the USB port and the midi port. It's not just a simple piece of cable. There are many of them, M-audio being one of the cheaper ones.
Yes, using a midi interface between your keyboard and PC lets you trigger and enter midi data into your PC. Your PC becomes the midi device while your keyboard the controller.
__________________
Kramer Striker USA, Epiphone Special II, V-H8, Soundgear Bass (Japan), Fender Squire MB-5, Aria Pro II Strat, EMG HZ, Seymour Duncan, POD 2.0, POD XTL, Badmonkey Tube Overdrive, Yamaha MSP5 http://www.facebook.com/group.php?gid=108888910190