In the roofing industry one square is equal to an area of 100 square feet.
A roofing square, a measurement of area, should not be confused with a framing square, a tool for making measurements.
Many roofing materials, like felt, are measured in their weight per roofing square. For example, material labeled "10# roofing shingle" would weigh ten pounds per square of roof.
Higher weights give you less squares per roll of shingle.
The most common concrete tile comprises about 90 tiles per roof square.