VLAN

VLANs segregate subnets, but subnets are not securely isolated.
VLANs make provisioning, routing subnets easier.
VLANs make moving physical subnets easier. They can maintain their virtual topology, but be physically relocated.
VLANs can be created at the layer 2 level (based on MAC addresses) which is simpler but can't cross switches.
VLANs can be created at the layer 3 level (based on IEEE 802.1q packet tagging) which allows more flexible routing over dumb switches.