Kempie has, as usual, provided an excellent answer.
I'll just add that the Visa symbol tells you that the card will work in any cash machine which also displays that symbol. In the UK that probably means every cash machine in the country but your card will also work throughout the world, to withdraw funds in the local currency, at any ATM with the Visa symbol. (In some countries that might not be all ATMs although, in most western european countries, the majority of ATMs accept Visa cards).
Chris