How does wireless 'hot spot' work?

I read a computer sales ad about getting the Internet access using the built-in wireless 802.11 connection. It also talks about hot spots where a wireless-capable computer can surfe the web when it's in a hot spot. What I don't understand is how much do I pay if I travel to UK (from USA) and use a hotspot at a cafe in London? Do I pay my US-based ISP or the cafe that provides the connection?