A fairy fort or fairy mound, known originally as a síd (plural síde) is the estate of a noble of the fae - essentially, it is the equivalent of a castle.
Specific Details
From the outside, a fairy fort appears to be a grassy burial mound. There are no visible doors or windows - to the untrained eye it could be a hill. However, if you know the magical 'password', of sorts, you can open a door in the side and enter the fort proper.
Yes, it is bigger on the inside. The interior is essentially a semi-sentient building, which can range in size from a small manor to an enormous palace depending on the power, status, and needs of the fort's master or mistress. The Seelie and Unseelie palaces are by far the most impressive of these, each containing essentially a small city's worth of rooms and people.
A fairy fort can change at the whim of its master/mistress - and can anticipate the needs of its inhabitants. For instance, if unexpected guests show up, a guest room might appear.
Used By
Aes Sídhe
All noble families of the aes sídhe have their own estates, which are passed down through generations (or may be won in battle).
Mna Sídhe
Some noble bean sídhe clans have forts, though they are just as likely to have more ordinary castles or homes.