This is really exciting stuff, kids. You may want to sit down.

802.1q is a standard for frame tagging, or frame identification. It was created to allow trunked links between disparate switches, so if you're using a Cisco switch with a different brand of switch, you would need to use 802.1q (and not ISL, the other type of identification used by Cisco). 802.1q inserts a field into the frame to identify a VLAN.