VLAN Trunking Protocol (VTP) overview
VTP (VLAN Trunking Protocol) is a Cisco proprietary messaging protocol used by Cisco switches to exchange VLAN information. VTP synchronizes VLAN information (such as VLAN ID or VLAN name) with switches inside the same VTP domain, which greatly simplifies network administration. A VTP domain is simply a set of trunked switches with the matching VTP settings (domain name, password and VTP version). All switches inside the same VTP domain share their VLAN information with each other.
To better understand the benefit of using VTP in your network, consider an example network with 100 switches. Without VTP, to create a VLAN on each switch, you would have to manually enter the VLAN creation commands on each switch! VTP enables you to create the VLAN only on a single switch. That switch can then propagate information about that VLAN to each switch in the same VTP domain, and cause other switches to create that VLAN too. Likewise, if you want to delete a VLAN, you only need to delete it on a single switch, and the change is automatically propagated to every other switch inside the same VTP domain.
The following network topology expains the concept more thoroughly.
In the picture above you can see a network of three switches. We have created a new VLAN on SW1. SW1 sends a VTP update to SW2, which in turn sends its VTP update to SW3. Now all three switches should have the same VLAN created.