A gatefold sleeve is a record cover that either opens like a book or folds out into three sections. Gatefold sleeves are sometimes used for double albums, and they are sometimes used so that the artwork or liner notes for an album need to be extended. For a single album, the record is usually kept in the right hand part of the gatefold sleeve, as you are facing it, and with double albums, one is kept in each side. When you close the gatefold cover, it looks like any other album.
Gatefold sleeves make an impact aesthetically, and often they are associated with special editions or collectable version of albums (although not always, many albums have a regular run of gatefold sleeves as well). When deciding whether or not to go gatefold, remember that they do tend to be more expensive. If you have the budget for one, it can look great, but you should compromise critical expenses like promotion to cover the costs of a gatefold sleeve.