A description of a parcel of land used to define the area for the purposes of ownership and rights - a metes and bounds description being one of the more common formats.
A legal description is used to define a parcel of land for ownership and rights transfer. The most common format is the metes and bounds description, which describes each boundary line segment in a series of calls traversing the edge. Metes and bounds are often referred to simply as legals, however there are other valid formats. See the Wikipedia entry for more detail.