Saint George is the patron saint of England, and other countries and regions as explained in the article on him. They also use the St George Cross, for example, in the flag of the city of Barcelona in Spain.
See Flag of the United Kingdom for the full explanation of the UK Union Flag and Flag of England for a bigger picture of the St George Cross, although as mentioned above, the St George cross is not just the flag of England.
St George's flag is not to be confused with the flag of the red cross. The flag of St George has a red cross which reaches from edge to edge of the flag. The flag of the red cross, like the Swiss flag, has a cross which does not reach the edges.