From what I gather it's an online currency, you purchase bit coins online then use them to buy stuff online. A bit like changing pounds to euros I guess but you can only use them online. So you go to bit coin website, pay I dunno say 40 quid from your bank card or paypal account, get 40 quids worth of bit coins in return and these sit in your bit coin wallet and you then use this to purchase from sites that accept them.