Pop music is a genre of music that covers a broad range of styles and is constantly changing to match the preferences of the majority. Since the 1950s, pop music has been written to access the widest audiences and sell the most copies.  It is usually characterised by a strong, simple rhythm in 4/4 time, simple, easily memorable tunes, and a structure where verses alternate with a repeated chorus. However, successful pop songs can be found that break some of these rules; for example, Biology by the British girl group Girls Aloud has a much more complicated structure than conventional pop songs, but is still considered pop because of its strong tunes and groovy rhythms.
In pop music, image is important. Singers are often chosen for their good looks and engaging personalities as well as, or even instead of, their singing ability. The most successful pop music artists pay very close attention to fashion and costume, staging, dance routines, packaging of their recorded music, and other elements that, while not directly related to the music in themselves, help to persuade fans to 'buy into' them as an entertainment package. However, this is not to say that pop music is all about image; many pop songs are musically excellent as well. A successful pop act usually needs to make good music and have an engaging public image.