Hey,
A MIDI controller is basically any piece of hardware which can transmit MIDI information. Most of the time when we talk about controllers, we mean keyboards. Most keyboards have sounds in them, but there are a few that don't have any sounds at all, all they do is send note information other MIDI devices like samplers and other synthesizers.
Think of a MIDI controller like a keyboard for your computer...by itself it doesn't do very much, but when you plug it into a computer, you can do all sorts of things (like type emails and play video games).
Take care,
Nick