Plain matrix:
\begin{matrix}
    1 & 2 & 3\\
    a & b & c
\end{matrix}

Matrix with parantheses:
\begin{pmatrix}
    1 & 2 & 3\\
    a & b & c
\end{pmatrix}

Matrix with square brackets:
\begin{bmatrix}
    1 & 2 & 3\\
    a & b & c
\end{bmatrix}

Matrix with curly brackets:
\begin{Bmatrix}
    1 & 2 & 3\\
    a & b & c
\end{Bmatrix}

Matrix with pipes:
\begin{vmatrix}
    1 & 2 & 3\\
    a & b & c
\end{vmatrix}

Matrix with double pipes:
\begin{Vmatrix}
    1 & 2 & 3\\
    a & b & c
\end{Vmatrix}