What is the difference between 'debt' and 'deficit'?

The UK's failure to meet a key public debt target "weakens the credibility" of its top AAA credit rating, the Fitch ratings agency has said.

Debt will now not fall as a proportion of the country's output until 2016-17, a year later than Chancellor George Osborne had targeted. But the budget deficit, which many analysts had predicted would be higher than it was last year, is now forecast to fall in 2012-2013.

The BBC's Simon Gompertz explains the difference between 'debt' and 'deficit'.