Slabs are buildings which serve primarily to commemorate the dead. Unlike coffins, slabs do not require any body part of the deceased to function, allowing you to prevent the appearance of ghosts of dwarfs whose bodies are no longer recoverable. Engraved slab memorials are usually made for your fort's citizens, but can be made in remembrance of any historical figure which dies in your lands.
A slab must first be constructed at a mason's workshop from a single stone, and can then be turned into a memorial at a craftsdwarf's workshop. After it has been engraved, the slab may be placed as a building. Make sure to ungroup the list when choosing an item for a building in order to prevent placing slabs which have not yet been engraved.
Information engraved on a slab can be viewed in the item description (for slabs placed as buildings, choose corresponding item from the building's Contained Items list). If you're not sure how a dwarf or creature died, engraving a slab in their memory may reveal the killer. You can use an engraved slab to create a memorial hall which functions identical to a sculpture garden.
Slabs, whether blank or engraved, can also be built solely to increase a room's value. They aren't terribly valuable but, unlike statues, don't block movement.