The Horcruxes are common items filled with a part of wizard’s soul. Making a Horcrux grants you immortality. If it’s so easy why doesn’t everyone do it? Because it is part of a black magic. To make a Horcrux and split your soul you must murder other human being. Black magic is forbidden and any form of it is considered evil. It is always used to hurt someone.
Lord Voldemort used black magic to make eight horcruxes and thus make himself immortal. That is as long as they are not destroyed. When Horcrux is destroyed, the soul within dies as well. When there is no soul left, the previous owner becomes mortal again. To destroy a Horcrux you must „damage it to a point where nothing, not even magic, can repair it” (for example using venom from basilisk’s fang or the Sword of Gryffindor).
1) Nagini – Voldemort’s snake. Killed by Neville in the last battle with Voldemort
2) Qurinus Quirrel – Proffesor of the Defence Against the Dark Arts. Killed by Harry Potter during his first school year
3) Marvolo Gaunt’s Ring – heairloom of the House of Gaunt, descendants of Salazar Slytherin. Destroyed by Albus Dumbledore
4) Slytherin’s Locket – heirloom of the Slytherin family. Destroyed by Ron Weasley
5) Ravenclaw’s Diadem – only known relic once belonging to Rowena Ravenclaw. Incinerated by Fiendfyre in room of requirements
6) Hufflepuff’s Cup – magical cup created by Helga Hufflepuff. Destroyed by Hermione in the chamber of secrets
7) Tom Marvolo Riddle’s Diary – diary of the Lord Voldemort himself. Destroyed by Harry Potter in the chamber of secrets
8) Harry Potter – he was a horcrux himself. Made in the moment his parents were murdered. He sacrificed himself to kill Voldemort and end his evil plans.