Sectumsempra (sec-tum-SEMP-rah) is a Dark Curse, invented by Severus Snape. It's first appearance was in the book Harry Potter and the Half-Blood Prince. When used, the opponent's chest, hands and face spurt out with blood, leaving the foe mortally scarred and injured. A counter-curse was also made by Snape, described as "an incantation that sounded almost like song".
Any body parts lost to this curse does not grow back. (George Weasley lost an ear to it)
Sectumsempra may come from the Latin sectum, meaning "having been cut", and sempra meaning "always".
The first use of this, as stated above, was in the half-blood prince. Harry Potter saw this curse scribbled in the margin of the princes copy of Advanced Potion Making, with text above stating "for enemies". Harry later used the curse on Draco Malfoy when he was crying in the bathroom, but Snape ran in and healed Malfoy's wounds just as Harry thought he was going to die.