You'll often see the term "a cord of wood" used. While many are unsure of it's exact meaning it is actually precisely defined:
Firewood stacked tightly to fill a volume of 4ft high, 4ft deep and 8ft long for a total of 128 cubic feet.
In the USA firewood can only legally be bought or sold by cords, or fractions of a cord. This protects both the buyer and seller from accusations of skullduggery and short selling. All the buyer needs to do to check the quantity of wood is to stack it tightly and check that the volume is correct.
I was kindly given permission to use some photos of Albert1029's, from over on the Hearth.com forums. In this picture you can see 1 measured cord of firewood, stacked in long rows, instead of a solid block.
The two rows at the back of the picture are each 12'x4'x16", for a total of 128 cubic feet. These are actually black locust, lucky for Albert as that is supposed to be some of the very best firewood possible! It is pretty rare over here in the UK, so I'm not expecting to get hold of some any time soon.
The firepit and seating area are pretty neat too.
Here is another one of the same stack from a different angle.
The defined volume, 128 cubic feet, is used as the standard measure for firewood because measuring by weight simply doesn't work. Imagine trying to buy a tonne of firewood - the seller has an incentive to sell wet, heavy unseasoned firewood, when the buyer actually wants the driest wood they can get.
The cord is also very easy to test - 4ft by 4ft by 8ft are simple lengths to measure and the volume is convenient when compared to how much wood people actually burn.
Sometimes you might here of "face cords", "ricks" or other non-standard measurements - if you are buying firewood do confirm in advance how many standard cords they mean, or get them to tell you the volume of wood.
Typically a home which is being heated with firewood throughout the winter will use between three and six standard cords of firewood, depending on climate, the stove and the building itself.
However, you need to get a few years ahead so that your wood has time to season properly. A well designed and properly sized firewood shed can help, we can season 3 years worth of wood in ours; 12 cords in total.
If you are just starting out consider buying kiln dried wood for you first year, but also buy in supplies now for the following year that you can leave to season.
At this point you may be feeling a little daunted - 12 cords of wood? How will I possibly find space for that in my garden? Stacked as individual cords you may struggle, but some methods get a lot of firewood into a compact space.