It is where you use the GPU compute capabilities of your graphics card to solve complex problems in order to generate a digital coin, the coin then can be sold for real money, some stores also accept Bitcoins and litecoins instead of real cash.
There are generally only a finite amount of coins that can be mined.

This is actually why AMD cards have such an inflated price currently, miners are buying them in droves to generate some extra money, AMD cards are better at it than nVidia cards. (A few places here in Australia are still selling out of high-end Radeons.)
Then market forces of supply/demand inflate the price.

Eventually it will reach a point where it's no longer economical to use GPU compute to mine for coins, the market will crash and we will have a massive influx of powerfull cards at a low price, which is going to be good news for me, I have multiple systems that could do with an upgrade. :P

alrighty then, i too look forward to that date lol

I was looking at HD 7850 1g vtx for my next pc in making, the cheapest is around £115 ish other manufacturers are a little more

now this is great news as the new r7 works out £110

it seems like excellent card for the money

VTX I am pretty sure is an offshoot to Powercolour.
They generally use better coolers in the lower/mid-range segments than other manufacturers just by simply using Arctic cooling solutions with a different sticker. :)

I've had better experience with VTX than say HIS over the years that's for sure, so nothing wrong with that choice.
If it's a reference design card, then they're all the same pretty much, only the warranty really differs.