MathCAD is a math worksheet software tool. Its strengths lie in long algebraic computations, vector/array calculation, unit tracking/conversion, equation solvers, and some basic programming.
Dr. Urroz has prepared an extensive page detailing MathCAD and its use which can be found at: http://www.neng.usu.edu/cee/faculty/gurro/Mathcad.html
A free software similar to MathCAD is SMath. Information about SMath can be found: http://smath.info/wiki/MainPage.ashx