In an Ethernet LAN a group of devices that receive a broadcast sent by any other device in the same group is called a broadcast domain. A VLAN is simply a defined subset of switch ports designated to be in the same broadcast domain.
Ports can be grouped into different VLANs on a single switch, and on multiple connected switches as well. Multiple VLANs create multiple yet contained broadcast domains. Any broadcast sent by one device on a VLAN is forwarded to all other devices on the same VLAN and is not forwarded to devices on other VLANs.
Best practices dictate a one to one relationship between VLANs and IP subnets. While it is possible to configure multiple IP Subnets on a single VLAN it is considered bad design to do so.
To forward frames between two devices in different VLANs a Multilayer Switch (MLS) or router is needed.