VTP (VLAN Trunking Protocol) is a Cisco proprietary protocol used by Cisco switches to exchange VLAN information. With VTP, you can synchronize VLAN information (like VLAN ID or VLAN name) with switches inside the same VTP domain.
To better understand the true value of VTP, consider an example network with 100 switches. Without VTP, if you want to create a VLAN on each switch, you would have to manually enter VLAN configuration 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 on a network and cause other switches to create that VLAN too. Likewise, if you want to delete a VLAN, you only need to delete it on one 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.
On SW1, we have created a new VLAN. SW1 sends a VTP update to SW2, which in turn sends its VTP update to SW3. Now all three switches have the same VLAN created.