The term singer-songwriter refers to specific groups of performers who both write and sing their own material. This distinguishes them from artists who are only singers, such as Elvis Presley, Dean Martin, Aretha Franklin, and Frank Sinatra, who typically sing the material of professional songwriters (who typically do not perform their own work, except as demos for the use of professional singers) such as Lorenz Hart, Richard Rodgers, Oscar Hammerstein, Cole Porter, Irving Berlin, Jerry Leiber or Mike Stoller. This arrangement --- singer and songwriter as discrete artists -- was the standard in popular music until about the 1960s.
In the late 1980s, the term was re-applied to a group of female singers and songwriters, beginning with Tracy Chapman, k.d. lang and P.J. Harvey. By the mid-1990s, the term's revival had grown to encompass Sarah McLachlan, Sheryl Crow, Lisa Loeb, Joan Osborne and Tori Amos, and other performers associated with the Lilith Fair.
Typically, a singer-songwriter will be a solo performer who accompanies him- or herself on guitar or keyboards, and will be equally well-known for the songs they write and for the way they sing them. The vast majority of singers who write their own songs -- Hank Williams, Muddy Waters, Harry Nilsson -- are not considered singer-songwriters.