BusyBox "The Swiss Army Knife of Embedded Linux" is a multiuse-utility, designed for embedded Linux-systems:

BusyBox combines tiny versions of many common UNIX utilities into a single small executable.
It provides replacements for most of the utilities you usually find in GNU fileutils, shellutils, etc.

A working system may consist of just a Linux kernel, some device nodes in /dev, a few configuration files in /etc, BusyBox, and maybe a bootmanager.

For example, BusyBox is used in Tiny Core Linux.

BusyBox can provide most of the functionality of the many programs typical found in /bin, /sbin, /usr/bin, /usr/sbin, /usr/local/bin, all in a single binary, thus saving space on small systems.

As a shell, BusyBox provides ash.

As an editor, BusyBox provides sed and Vi.

BusyBox can be configured to include as little or as much "applets" as desired.

BusyBox can be compiled to include an AWK-implementation.