matrices is about matrices in equations and diagrams.

Using AMS-LaTeX (package amsmath, the AMS document classes, etc.), there are several commands to create matrices in math mode. For example,

I_2 = \begin{bmatrix}
  1 & 0 \\
  0 & 1
\end{bmatrix}

typesets:

There are many other commands available to typeset matrices:

• The bmatrix environment, shown above, typesets a matrix in braces. matrix typesets a matrix with no delimiters; pmatrix uses parentheses, Bmatrix uses braces, vmatrix uses vertical lines, and Vmatrix uses double vertical lines.
• The matrix* environment allows one to specify the alignment of the matrix elements: for example, \begin{matrix*}[r]...\end{matrix*} right-aligns all of the columns. There's also pmatrix*, bmatrix*, etc.
• The smallmatrix environment typesets a matrix in line, and is small enough to look good in text-style math. In addition, the mathtools package provides psmallmatrix, bsmallmatrix, etc., which also add delimiters on each side.
• The bordermatrix command allows one to annotate the rows or columns of a matrix (also see the kbordermatrix package).
• TikZ has a matrix library which allows extensive customization. If your question is about this library, use the tikz-matrix tag.

Frequently asked questions:

• What's the best way to make an augmented "coefficient" matrix?
• How can I increase the line spacing in a matrix?
• How can I highlight part of a matrix?
• Where is the \matrix command?