A felony is crime considered serious, generally punishable in the United States by one year or more in prison or (rarely) by death. Murder, for instance, is a felony. Lesser offenses are usually called misdemeanors. An example of the distinction is in Wisconsin, where it is a misdemeanor for an adult to have sexual relations with a minor 16 years or older, and it is a misdemeanor for an adult to have sexual relations with a minor younger than 16. The second act is considered far more offensive than the first. In Ohio possessing certain types of controlled substances is a felony. 
Each year new laws invent many new crimes as felonies, even though for thousands of years they were not even considered to be criminal. For example, in some cases draining water from your property without a government permit or removing asbestos from your property without government authorization may be a felony. Such "crimes," which affect nobody but the property owner, have only under recent, liberal administrations become the government's business.