The Cave

The Horcrux cave was the cave where Albus Dumbledore and Harry Potter traveled to in the spring of 1997, in search of Salazar Slytherin's Locket which was one of Lord Voldemort's Horcruxes. It was a seaside cave located somewhere in Britain.

Early History
Before he became Voldemort, Tom Riddle once visited this cave, with his orphanage. During the trip he had terrorized fellow orphans Amy Benson and Dennis Bishop in this cave.

Horcrux hiding place
he placed on it a very large number of complex enchantments, designed to keep others out but allow him access to the Horcrux:

After swimming to the entrance, Dumbledore spatters it with his blood to gain entry and they take a hidden boat across a lake. Protecting the Horcrux there is a potion that Dumbledore drinks, though Harry must force him to; after they obtain the locket Inferi erupt out of the water, and a weakened Dumbledore casts a fire spell to repel them as the pair flees the cave.