So, I was lurking in a music store and came across some USB controllers. They really interested me, as they seemed to be quite cheap compared to regular keyboards or synthesizers.

I don't really know anything about keys, but I'd like to learn. So, for those who know about the subject, what are the differences between usb controllers/synthesizers/keyboards?
do you mean what is the difference between a USB controller, a synth, and a keyboard?

a USB controller is a MIDI controller that hooks to other devices (such as a computer) via USB. it has no sounds of it's own, it only controls other devices.

a synthesizer is an electronic device made to create sounds. they have varying controls that allow quite a bit of tweaking of the sound. hardware synths either come in modular (just a sound module, which requires a controller) or keyboard form (keys and engine in one unit), though sometimes they take other forms. software synths are synth programs for computers, which can be controlled by an external controller.

keyboard can refer to keyboard instruments in general, but generally refers to an electronic keyboard, which come in a wide variety of types, such as portables, arrangers, workstations, digital pianos, etc.

if you have any other questions, feel free to inquire in the Keyboard Thread (link in sig).
There's no such thing; there never was. Where I am going you cannot follow me now.
If you're referring to a midi controller, it won't really be similar to a keyboard or synthesizer. A midi controller is used to control the parameters of another program/utility by 'mapping' the program's functions to the midi controller's interface. So in short it is just an interface used to control other devices.

edit: d'oh beaten to it ^ his answer is better and more complete so feel free to ignore this lols