An aria is usually a self-contained vocal piece, which may be part of a larger work. The typical context for arias is opera, but they may also be found in cantatas, oratorios, or similar stage works. Arias are commonly classified as either 'binary' or 'da capo', a distinction in reference to their form. The da capo aria features a highly ornamented repetition of the opening section, which was often used as a way of displaying the technical skills of the singer. Da capo arias also featured a recurring instrumental section, the ritornello, which some believe to be an important factor in the later development of the concerto and sonata forms. The reforms that Christoph Gluck introduced in the opera genre included a heightened focus on drama and passion, which required the da capo section of arias to be treated in a subtler way by singers in order to keep the melody recognizable and avoid drowning the narrative.
Sometimes arias are composed as stand alone pieces, or written as specific replacements for other arias, in which case they are labeled 'concert arias'. The term 'aria' is also used to refer to some 17th century instrumental genres, usually followed by a set of variations (a late example of this is the set of Goldberg Variations by J.S. Bach, of which the 'theme' is titled ARIA)