I've never used or recommend Wonga to anyone, but reading from cazzz's link above, if, for example, you need £100 urgently 'today', have no alternative means of getting it, and can pay it back in a few days, Wonga will lend you it and after 5 days you'll pay them £110.70.
A fee of £10.70 over 5 days to get you out of a temporary hole if you have no other option, doesn't seem that bad imo. It's only if you don't pay back what you owe, when required (and previously agreed) that you get stung big time...