A song is a vocal musical form that can involve one or many performers, usually (but not always) involving a text or words. Songs are one of the most popular forms of musical expression, and as such there are many different traditions, genres, and classifications for songs. Songs in Western classical (academic) music are often referred to as 'art songs', and they are usually conceived for concert performance. They commonly involve poetical text, and require a considerable level of vocal technique. The art song as a genre has developed in different regions and countries, branching into several denominations. In general terms, the European art song is considered to come from both medieval secular bard music, and religious compositions.