Now integrated in the LaTeX kernel as `ltcmd`. Formerly, `xparse` was high-level package from the LaTeX3 project that provided a powerful mechanism for defining new commands with various types of arguments. The current `xparse` package gives access to deprecated command types and its usage is not recommended.
The main interface implemented in `ltcmd` is the `\NewDocumentCommand` macro and its variants, which improve on `\newcommand` and its variants, allowing commands with multiple optional arguments of various formats, verbatim arguments, and so on.