A cord is defined as a volumetric unit equivalent to 128 cubic feet (4'X4'X8'). Many wood dealers sell by the pickup load which results in widely variant quantities of wood, usually considerably less than a cord.

When generating heat by means of a wood fire, it is noteworthy to consider the widely variant heat value of different tree species.  As a rule, you'll get best results burning the highest density (heaviest) hardwood with the highest BTU (available heat) content that can be found in your area. 

Seasoned (cut and dried for at least one year) black locust, at 26.8 MBTU per cord, is far superior to both white oak (25.7 MBTU/cord) and red oak (24 MBTU/cord) varieties. The only thing better is Ironwood, Hickory and Hedge. None of which are available regionally. 

Regardless of what species of wood you burn, it won't produce optimal heat output and burn time unless it is properly seasoned. You can read more about why you shouldn't burn unseasoned fuel wood here.   

We take our heat energy data from “The Chimney Sweep, Inc.” http://chimneysweeponline.com/howood.htm