Daz, the grey thing at the bottom is the tap, similar to a Bitburger keg. There is a gas cartridge built in, you have to activate it by turning the red lever on the top. There is also a little plastic cap on the top, take out, fill with carbed beer from keg (hose to the bottom), and replace plastic cap with sturdy bung or whatever you wanna call it that comes with it. Once gas is activated and beer empty, the gas is obviously gone, so not perfect for re-use.
Advantage is that you don't have to take your gas setup to a party, but it's a bit of an expensive exercise if you want to drink a bit.
I use them for presents, (christmas, birthdays, client gifts etc.), always a hit if the beer you put in lives up to it.