The SYN, ACK and SYN/ACK form the three-way habdshake that computers use to identify and connect to each other. It is used in SYNflooding (or is it DOSing) by sending high number of SYN requests to the target computer, which answers back with the SYN/ACK and waits some time for the SYN/ACK that's never coming. Due to the high numbers of requests the queue gets longer and eventually takes up the whole bandwidth.
Anybody correct me if I'm wrong.