MatrixForm[list] prints with the elements of list arranged in a regular array.
In StandardForm the array is shown enclosed in parentheses.
MatrixForm prints a single-level list in a column. It prints a two-level list in standard matrix form. More deeply nested lists are by default printed with successive dimensions alternating between rows and columns.
Elements in each column are by default centered.
MatrixForm takes the same set of options as TableForm.
MatrixForm acts as a "wrapper", which affects printing, but not evaluation.
See The Mathematica Book: Section 1.8.3 and Section 2.8.8.
See also: TableForm, ColumnForm, GridBox, GraphicsArray.