A virtual LAN (VLAN) is any broadcast domain that is partitioned and isolated in a computer network at the data link layer (OSI layer 2) .

Source: https://en.wikipedia.org/wiki/Virtual_LAN

Enables you to segment a physical network into multiple discreet networks without having additional hardware.

• Create a VLAN by taking a single physical broadcast domain made up of one or more switches and chop it up into multiple broadcast domains.
• Requires switches with specific programming to create the virtual networks.
• Typically a single port will be assinged to a specific VLAN
• Can use multiple switches connected to multiple VLANs through a process called trunking .