I've mentioned open ports. I guess I'll explain this a bit more because this is something you should probably know before getting into the game.
Although Nmap is now one of the best scanning software out here, it started as a port scanner. It's a simple but efficient port scanner. The simple command
nmap on target scans 1000 TCP ports on it. You've probably heard of open or closed ports, but in fact Nmap divides ports into six states:
First, let's explain open and closed ports. If a host is accepting Transmission Control Protocol (TCP) connections, User Datagram Protocol (UDP) datagrams, or Stream Control Transmission Protocol (SCTP) associations, we call this port opened.