This is a 49 key USB midi keyboard with 'hard' modulation and pitch bend controls, a semi-soft volume control, and soft octave switchers. It's very compact, with just about 2 inches more depth than the keys themselves, which are full-sized, and about one inch of casing at the top end, and space for the controls at the bottom. It doesn't require any power because it's powered from the USB cable, and has an input for a sustain pedal.
Some clarifications, because the description on Amazon doesn't make it quite clear:
There are no 5-pin DIN midi outputs on this keyboard -- it communicates midi, but solely through the USB connector.
Likewise, there is no onboard sound generation -- it's all done on your computer using your sampler, soft instruments, etc.
The keys are 'synth-type', which means they are velocity sensitive but not hammer action (or even pseudo-hammer action) like a piano. As far as I can make out (both from testing and the specifications) there is no 'after-touch'.
There is a USB cable in the box, which is long enough for a fairly close computer, but definitely not to cross a room.
As this is USB powered, you can plug it directly into a USB 2 computer, or into a powered hub, but not into an unpowered hub or the USB port on your computer keyboard.
There is no paper manual -- it's all fairly self explanatory, but you can download the manual from M-Audio's website, or pull it off the CD.
So, what's this like, and what's it for? This is a composer, song-writer or sequencer's tool for inputting midi into Logic, Cubase, and other midi-compliant software on a Mac or a recent Windows PC. Allegedly both systems will instantly detect the midi instrument without having to install the supplied software. For Mac Snow Leopard with Logic, this is more or less true: Logic correctly identified the keyboard and recognised the octave and level settings, but did not recognise the modulation or pitch bend (or maybe it was just switched off on that sound -- but, either way, it didn't pick it up out of the box). For a Midi device, this is pretty good, as you generally have to supply information about the keyboard for it to pick up all the functions. More importantly, the system understood the velocity sensitive keys.
You can install the supplied software if you want -- I don't, as it's largely 'lite' versions of things.
I noticed a small amount of latency (delay between playing and the sound) -- more than on a pro keyboard. I wouldn't suggest using this for a live performance, except as an additional controller for effect sounds. That's not what it's for, though. What it is for is sequencing a track or inputting a score. It's very handy for that, light enough to put away and small enough to store easily. 49 keys is also fine for this purpose.
Finally, M-Audio is a high-end prosumer supplier, owned by Avid, the parent company of Sibelius and Pro-Tools. M-Audio gear tends to be pro-useable quality, but without the refinements of Pro-Tools. It's at the upper end of the space occupied by Alesis, Tascam and other prosumer suppliers, and substantially above Behringer, Zoom and the other entry level 'serious' brands.