[tex]\left(\begin{array}{ccc}a & b & c \\d & e & f \\g & h & i\end{array}\right) [/tex]
Gives
If you use pmatrix, you don't have to insert the parentheses "by hand".
[tex]\begin{pmatrix} a & b & c \\ d & e & f \\ g & h & i \end{pmatrix}[/tex] gives .
For a bracketed matrix, use bmatrix. For a determinant, use vmatrix.
[tex]\begin{bmatrix} a & b & c \\ d & e & f \\ g & h & i \end{bmatrix}[/tex] gives .
[tex]\begin{vmatrix} a & b & c \\ d & e & f \\ g & h & i \end{vmatrix}[/tex] gives .