Netcat is a network debugging and investigation tool that can assist with port scanning, transferring files, and port listening, and it can also be used as a backdoor. Many descendants.
Ncat is an implementation of the Netcat (nc) tool for Nmap. Useful for redirecting TCP and UDP ports to other sites, and supports SSL and proxy implementations with SOCKS.
Netcat (nc) is a tool that is distributed with many Linux distributions for tasks with TCP or UDP. It is scriptable using Bash, and can be used to transfer files to or from target machines, implement bind or reverse shells, or directly test a service using a raw connection.