Area Command

Area( <Point>, …, <Point> ): Calculates the area of the polygon defined by the given points.

Area((0, 0), (3, 0), (3, 2), (0, 2)) yields 6.

Area( <Conic> ): Calculates the area of a conic section (circle or ellipse).

Area(x^2 + y^2 = 2) yields 6.28.

Area( <Polygon> ): Calculates the area of the polygon.

for Polygons, the absolute value of the Algebraic Area is calculated (which gives unexpected answers for self-intersecting polygons)
In order to calculate the area between two function graphs, you need to use the command IntegralBetween.
See also the Area tool.