Hogwarts was a school dedicated to teaching young wizards and witches the art of magic. It was located in Scotland and could be reached from London with a magical train that left from a hidden platform on King's Cross Station.
Hogwarts was a huge castle. The school had numerous charms and spells on and around it that make it impossible for a Muggle (i.e. a non-magical person) to locate it. The castle had extensive grounds with sloping lawns, flowerbeds and vegetable patches, a loch (called The Black Lake), a large dense forest (called the Forbidden Forest), several greenhouses and other outbuildings, and a full-size Quidditch pitch. There was also an owlery, which houses all the owls owned by the school and those owned by students. Some rooms in the school tended to "move around", and so did the stairs in the grand staircase. Hogwarts was divided into four houses, each bearing the last name of its founder: Godric Gryffindor, Salazar Slytherin, Rowena Ravenclaw and Helga Hufflepuff.
The Antichrist also attended Hogwarts, but when he discovered his true nature and how everyone around him had been manipulating him, he went on a murderous rampage, killing all the students and staff of the school. His attack apparently also undid the protection spells that keep muggles out, because in 2009 Mina Murray and Orlando were able to visit Hogwarts when they wanted to learn more about the Antichrist. They used the still functioning remains of the Hogwarts Express to get there. Once they arrived at Hogwarts, they found a completely devestated castle filled with the skelletal remains of the students and staff.
Hogwarts is the main setting for the Harry Potter novels by J.K. Rowling.
Because Harry Potter-media is still very much in copyright, Hogwarts is not mentioned by name in any LoEG media, but it is clear that it is indeed the magic school from Harry Potter. Hogwarts is named as the "invisible college".
In the Harry Potter books, it is stated that non magical people cannot see the school; rather, they see only ruins and several warnings of danger. This is exactly what Mina and Orlando see when they visit the school, except that by then the school really has become a set of ruins.