The short answer is that a square is a 100 square feet. So as an example, if a house has a 32000 square, it would be 320 squares.
In terms of application, here are a few interesting points to keep in mind about the unit of Square. Adjusters from insurance companies use the unit of a square to test the integrity of a 100 square feet area, sometimes also called "test square". If in that area there are more then 10 hail hits, or other clear issues that effect the integrity of the roof, the insurance company will replace or repair the roof area tested. They actually test all sides of the roof -- North, South, East, and west. If three sides are damaged then the whole roof will be replaced.
Also it takes three bundles of shingles to cover one square. So if a house is 20 squares, or 2000 square feet, it will need 20 x 3 to cover for 20 squares. (in reality its more, but this is just to demonstrate.
So, here is some more math for squares
What is a Roofing Square?
The term "roofing square" and "roof pitch" are the most commonly used terms in the roofing industry.
1 roofing square = 100 square feet. It does not matter how you arrive at 100 square feet
10 feet x 10 feet = 100 square feet or 1 roofing square
5 feet x 20 feet = 100 square feet or 1 roofing square
2 feet x 50 feet = 100 square feet or 1 roofing square
50 feet x 50 feet = 2500 square feet, so 2500 / 100 = 25 roofing squares
Bundles of roofing material per square
Most composition shingles are 3 bundles per square
Most of the 40yr composition shingles are 4 bundles per square (Some 5 or more)
Wood Shakes 5 bundles per square
Wood Shingles 4 bundles per square Roofing felt per square Roofing felt comes in a wide variety coverage's.
Felts (Sqs per roll)
Some of more popular felts #15 lb felt; 4 squares per roll (Some 3 squares per roll)
#30 lb felt; 2 squares per roll
#30 lb 18" shake felt 1 square per roll
Fiberglass base sheet 1 square to 3 squares per roll
Cap Sheet 1 square per roll
Modified bitumen 1 square per roll
Roofing tiles have a vast difference in how many pieces per square
The more popular concrete tiles around 90 pieces per square
Clay 1-piece (or S Tile) about 80 to 100
Clay 2-piece 160 and up