What is per vlan spanning tree

Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network.
It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs.
Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another trunk without causing a Spanning Tree loop.
source:http://www.cisco.com/en/US/tech/tk389/tk621/tk846/tsd_technology_support_sub-protocol_home.html

Per-VLAN Spanning Tree One of the things that must be considered with VLANs is the function of the Spanning Tree Protocol (STP).
STP is designed to prevent loops in a switch/bridged topology to eliminate the endless propagation of broadcast around the loop.
With VLANs, there are multiple broadcast domains to be considered.
Because each broadcast domain is like a unique bridged internetwork, you must consider how STP will operate.
The 802.1Q standard defines one unique Spanning Tree instance to be used by all VLANs in the network.
STP runs on the Native VLAN so that it can communicate with both 802.1Q and non-802.1Q compatible switches.
This single instance of STP is often referred to as 802.1Q Mono Spanning Tree or Common Spanning Tree (CST).
A single spanning tree lacks flexibility in how the links are used in the network topology.
Cisco implements a protocol known as Per-VLAN Spanning Tree Plus (PVST+) that is compatible with 802.1Q CST but allows a separate spanning tree to be constructed for each VLAN.
There is only one active path for each spanning tree; however, in a Cisco network, the active path can be different for each VLAN.

Per-VLAN Spanning Tree (PVST) maintains a spanning tree instance for each VLAN configured in the network. If there is100 VLAN there should be100 STP process 

simple and short in one line single instence for single VLAN : or per instence for per vlan