Identity Matrix (I)
$$\begin{pmatrix} \textcolor{blue}{1} & \textcolor{gray}{0} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{blue}{1} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{blue}{1} \end{pmatrix}$$
Diagonal of ones, zeros elsewhere. Special case of diagonal, scalar, and symmetric matrices.
Zero Matrix (O)
$$\begin{pmatrix} \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{gray}{0} \end{pmatrix}$$
All elements are zero. Singular matrix with determinant 0 and rank 0.
Diagonal Matrix
$$\begin{pmatrix} \textcolor{blue}{5} & \textcolor{gray}{0} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{blue}{-3} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{blue}{2} \end{pmatrix}$$
Non-zero entries only on main diagonal. Easy to compute powers and determinant.
Symmetric Matrix
$$\begin{pmatrix} \textcolor{blue}{4} & \textcolor{green}{1} & \textcolor{green}{2} \\ \textcolor{green}{1} & \textcolor{blue}{3} & \textcolor{gray}{0} \\ \textcolor{green}{2} & \textcolor{gray}{0} & \textcolor{blue}{5} \end{pmatrix}$$
A = Aᵀ. Real eigenvalues, orthogonal eigenvectors. Common in physics and optimization.
Upper Triangular
$$\begin{pmatrix} \textcolor{blue}{2} & \textcolor{green}{4} & \textcolor{green}{1} \\ \textcolor{gray}{0} & \textcolor{blue}{3} & \textcolor{red}{-1} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{blue}{5} \end{pmatrix}$$
All entries below diagonal are zero. Determinant = product of diagonal entries.
Lower Triangular
$$\begin{pmatrix} \textcolor{blue}{3} & \textcolor{gray}{0} & \textcolor{gray}{0} \\ \textcolor{red}{-1} & \textcolor{blue}{2} & \textcolor{gray}{0} \\ \textcolor{green}{4} & \textcolor{green}{5} & \textcolor{blue}{1} \end{pmatrix}$$
All entries above diagonal are zero. Used in LU decomposition.
Orthogonal Matrix
$$\begin{pmatrix} \textcolor{gray}{0} & \textcolor{blue}{1} & \textcolor{gray}{0} \\ \textcolor{blue}{1} & \textcolor{gray}{0} & \textcolor{gray}{0} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{red}{-1} \end{pmatrix}$$
AᵀA = I. Preserves lengths and angles. Represents rotations/reflections.
Singular Matrix
$$\begin{pmatrix} \textcolor{blue}{2} & \textcolor{green}{4} & \textcolor{green}{6} \\ \textcolor{blue}{1} & \textcolor{green}{2} & \textcolor{green}{3} \\ \textcolor{gray}{0} & \textcolor{gray}{0} & \textcolor{gray}{0} \end{pmatrix}$$
det(A) = 0. Not invertible. Rows/columns are linearly dependent.
Column Stochastic
$$\begin{pmatrix} \textcolor{green}{0.5} & \textcolor{green}{0.2} & \textcolor{green}{0.3} \\ \textcolor{green}{0.3} & \textcolor{green}{0.5} & \textcolor{green}{0.3} \\ \textcolor{green}{0.2} & \textcolor{green}{0.3} & \textcolor{green}{0.4} \end{pmatrix}$$
Non-negative entries, each column sums to 1. Used in Markov chains.
Rectangular Matrix
$$\begin{pmatrix} \textcolor{blue}{1} & \textcolor{green}{2} & \textcolor{green}{3} \\ \textcolor{green}{4} & \textcolor{blue}{5} & \textcolor{green}{6} \end{pmatrix}$$
Rows ≠ columns. No determinant or eigenvalues, but has rank and singular values.