XAML-Math supports multiple types of matrices and similar constructs (that allow to build the items into rows and columns).
All commands offer a similar syntax, but a bit different features. Rows inside of a matrix are separated by two commands:
\cr\\
Cells inside of a row are separated by &.
For example, the following command creates a matrix with 2 rows and 3 columns:
\matrix{1 & 2 & 3 \\ 4 & 5 & 6}
There're two matrix types supported:
\matrix: a matrix without brackets\pmatrix: a matrix within square brackets
There's also a matrix-like construct:
\cases: for piecewise functions etc
See also the documentation on environments.