Lieder, the plural form of Lied, is literally the German word for "song." In classical music, the term is specifically used for songs usually set to German poetry, written largely by German or Austrian composers in the late 18th and 19th centuries, which attempt to illuminate the text's meaning through musical means. Lieder were typically written for solo voice with piano accompaniment, though late Romantic composers also wrote lieder with orchestral accompaniment. In English, the term is sometimes translated as "Art Song."
While Mozart and Beethoven wrote famous early examples of lieder, the genre fully blossomed during the early 19th century in the music of Franz Schubert, who wrote over 600 of them. Later, Robert Schumann and Johannes Brahms devoted special attention to the genre, and Hugo Wolf devoted his entire career to them. Often, the texts of lieder can describe an idyllic setting, as in Gustav Mahler's "Die Knaben Wunderhorn" or Franz Schubert's "Die Schöne Müllerin," "Fischerweise," or "Erlkönig." The songs retain much of their popularity today, and are widely performed by operatic soloists.
Often, composers grouped lieder into cycles, which are termed "Liederkreis," or song cycles.
James Parsons, The Cambridge Companion to the Lied, 2004.