Subgenre: A more specific category within a larger musical genre. For example, punk rock is a subgenre of the broader genre of rock.
Crossover: When a musical genre combines elements from two or more different genres. An example would be when country music blends with pop music to create a crossover hit.
Mainstream: The popular and widely accepted styles of music at a given time. Mainstream genres often dominate the charts and receive the most airplay on radio stations.