Trunking - in a VLAN concept, this means carrying multiple VLANS (re: Different) through one VLAN link. This is done via the 802.1Q standard which dictates that a tag be added to each Ethernet header to differentiate it from another VLAN.
Cisco also has it's own means of do this by encapsuling a frame into a container, with the container being sufficient to identify the VLAN that the frame is associated with. This is called "Inter-Switch Link". Obviously this technology is only supported by Cisco devices.