What is a matrix?

A matrix is a series of mathematical expressions arranged in a rectangular form between square brackets or parenthesis'. The expressions can be numbers or variables. Size of a matrix can determined by the number of rows and columns. This can also be expressed in both numerical and variable format.

Other types of matrices: vectors.
A row vector is a matrix with only one row. (1xn)
A column vector is a matrix with only 1 row. (mx1)