To add to ethel's reply.
Generally when you take out a card, your spending limit will be agreed at the outset, however, some companies will increase this if you reach the limit - not necessarily a good thing.
basically, if you are really well organised, like Ethel, then they are undoubtably an asset.
If you are not, then you run the risk of getting into debt very quickly - the interest on the cards (apart from introductory offers) tends to be higher than a loan and if you pay only the minimum, you could be paying back triple what you spent.
I have a credit card, but I use it for buying things on-line and for large items for the insurance, I do however pay it off monthly and it costs me nothing.
If I needed a large item urgently, and didn't have the money to pay for it, i would take out a loan and buy the item with my credit card and repay the card with the loan, then you have the insurance protecion and you have to repay at a fixed rate and are commited to paying it back in the time you intended.