Vis is the energy that powers all thaumcraft devices and is required to craft the majority of them as well.
When you use your wand your wand uses the Vis that it had charged up, after which it will take the vis from the area to recharged.
Vis is stored in nodes that can be seen with the goggles of revealing.
For more information on nodes see Node.
When vis is used, Flux is released into the enviroment.
See Flux Reducing Ideas to help prevent flux from getting out of control.