“Meterpreter is an
advanced, dynamically extensible payload that uses in-memory DLL
injection stagers and is extended over the network at runtime. It
communicates over the stager socket and provides a comprehensive
client-side Ruby API. It features command history, tab completion,
channels, and more.”

Here is a wonderful
tutorial on using Metasploit to create an EXE with the Meterpreter
payload that creates a reverse TCP connection outbound, blowing
through many NAT boxes and firewalls.