A midi controller is basically a keyboard with no sound. Normally, you would have it as a keyboard, but it could also take the form of a drum kit, a guitar, or a trigger...
Basically, the midi controller sends out infos to a midi device. When I say midi device, I mean a machine that can support midi inputs.
When you play it, the infos on the note you want to play (pitch, length, volume, modulations...) are read my the midi device and you hear the sounds of the midi device itself. Since Reason can accept midi inputs, that means that you can play all the sounds in Reason with the midi controller...
So, to sum everything up, if you want to play your beats (in real time) with a keyboard, a midi controller IS what you need. What you would have to look for is what kind do you need (number of keys, other accentuators like the pitch bend, modulation wheel and other knobs that can send out other infos to Reason...)
http://www.m-audio.com/index.php?do=products.list&ID=midikeyboardcontrollers
M-Audio got some real nice controllers
http://www.m-audio.com/products/en_us/TriggerFinger-main.html
That's a midi trigger finger, for the drums... if you want...