A championship is a contest where a champion — an ultimate winner — is decided.
Champions are the top winners: only one football or baseball team earns the title of champion in a year. The championship is the contest that decides who will be champion. The Super Bowl is a championship. And so are the World Series, the Stanley Cup Finals, and the NBA Finals. Although championship is most often used in sports, you will also hear about the spelling bee championships or chess championships. When you win a championship, you're number one in your field.