“A SYN packet is sent (as if we are going to open a connection), and the target host responds with a SYN+ACK, this indicates the port is listening, and an RST indicates a non- listener. The server process is never informed by the TCP layer because the connection did not complete.”
http://www.auditmypc.com/port-scanning.asp
I did a bit of research and this is what I found throughout the internet via google.