AWK is a programming language. There are several implementations of AWK (mostly in the form of interpreters). AWK has been codified in POSIX. The main implementations in use today are:
* `nawk` (“new awk”, an evolution of `oawk`, the original UNIX implementation), used on *BSD and widely available on Linux;
* `mawk`, a fast implementation that mostly sticks to standard features;
* `gawk`, the GNU implementation, with many extensions;
* the Busybox (small, intended for embedded systems, not many features).
If you only care about standard features, call `awk`, which may be Gawk or nawk or mawk or some other implementation. If you want the features in GNU awk, use `gawk` or Perl or Python.