Its name means slow (and) violent, as this style typically has a tempo between 85 and 115 beats per minute (BPM).
It consists of a hard kick, like the ones present in hardcore or hardstyle, but played at a very slow tempo, with vocal samples and dark and acid sounds or loops.
While some individual tracks were popular in the clubs, the style itself was not established until DJ/producer Gigi D'Agostino started with a series of productions in 2003, like "Ripassa".
Indeed, he coined the name for this style, and he releases most of his lento violento songs on his own label Noise Maker, using aliases like Dottor Dag, Lento Violento Man, Uomo Suono, Orchestra Maldestra, or La Tana Del Suono, to distinguish these songs from his Italo dance work.
Also several compilation CDs are released with lento violento music, like the Movimentolento series.