From Wikipedia:
http://en.wikipedia.org/wiki/40_acres_and_a_mule40 acres and a mule is the colloquial term for compensation that was to be awarded to freed American slaves after the Civil War—40 acres (16 ha) of land to farm, and a mule with which to drag a plow so the land could be cultivated.
The award—a land grant of a quarter of a quarter section deeded to heads of households presumably formerly owned by land-holding whites—was the product of Special Field Order No. 15, issued January 16, 1865 by Maj. Gen. William T. Sherman and applied to black families who lived near the coasts of South Carolina and Florida (the original order specifically allocated "the islands from Charleston, south, the abandoned rice fields along the rivers for thirty miles back from the sea, and the country bordering the St. Johns river, Florida"). There was no mention of mules in Sherman's order, although the Army may have distributed them anyway.
Some people mistakenly believe that President Andrew Johnson vetoed the enactment of the policy as a federal statute (introduced as U.S. Senate Bill 60), although this vetoed Freedmen's Bureau bill, in actuality, made no mention of grants of land or mules. (Another version of the Freedmen's bill, also the land grants, was later passed and Johnson's second veto overridden.)
Few if any former slave families ever took possession of the land or the apocryphal mule, and the phrase has come to represent the failure of Reconstruction and the general public to assist African-Americans in the path from slavery to freedom