@lightning88: A MIDI keyboard is an instrument designed to work with external sound sources or computer software. It would typically have generic controls (knobs, buttons, sliders) that can be assigned to different aspects of the software, such as track volume, sound effects, and even editor/sequencer transport controls (play, stop, record, etc.).
A MIDI keyboard usually doesn't have any on-board sounds, so it isn't a synthesizer. However, there are many keyboard synths can that serve nicely as a MIDI keyboard. A musician would typically have a master MIDI keyboard in a setup where he/she uses many different types of software (and hardware rack-based) synthesizers in their studio, or performing live, when most of their synths are running on a laptop computer, or in a rack.
I'm not clear on the term "virtual keyboard synth," but it may refer to "virtual analog," a marketing buzzword for a keyboard synthesizer that uses a modern digital architecture to produce sound, but functions as if it was based on an analog architecture. It may also mean a "softsynth," a synthesizer that runs on a computer, and acts like a hardware synth, but you need a MIDI keyboard (or MIDI information) to play it.