Hogwarts, or more formally Hogwarts School of Witchcraft And Wizardry, is a school that teaches children of wizarding families. The school is a large castle perched on a mountaintop near a lake.
Students at Hogwarts are expected to wear robes, and forbidden to go to the Forbidden Forest or practice magic in the corridors between classes. The students are divided into four houses--Gryffindor, Hufflepuff, Ravenclaw and Slytherin. Each house has its own residence, e.g. Gryffindor Tower, guarded by a special painting that will only let someone in if they know the password--and if the painting's inhabitant is present. Each house accumulates points towards an annual house cup; points may be awarded to a house by professors to reward good performance by the students, or taken away to penalize them for poor performance or rule-breaking. Hogwarts is also populated by many ghosts, some of which are officially associated with one of the houses.
First-year students are brought across the lake in magical boats, then into the Great Hall for the Sorting. There is a hole in the cliffside where the boats can enter, go through a tunnel, and into an underground harbour. From the harbour there are stairs leading up to huge oak front door of the castle. The doors lead to the entrance hall; on the right is a large doorway into the Great Hall, though first-years are led to smaller chamber to prepare for the Sorting.
Staircases lead up to the upper floors, or down to the dungeons. There are 142 staircases at Hogwarts, some of which have trick steps and some of which don't always lead to the same place. Doors in Hogwarts can also be tricky, some needing to be tickled or asked politely, and some never opening at all. The corridors are decorated with portraits and suits of armour, but the portraits move around and it is thought that the suits of armour do, as well. There is a trophy room on the third floor, which is never locked.
Outside the castle is the Forbidden Forest, and behind it are the greenhouses where Herbology is taught. Rubeus Hagrid, the Gamekeeper, lives in a shack on the edge of the forest. On the opposite side from the Forbidden Forest there is a grassy slope down to a smooth lawn where flying lessons are held.
Also outside the castle is the Quidditch pitch. There are hundreds of seats in raised stands, and three fifty-foot tall goalposts with hoops at the top at either end.