Vis is a form of power introduced by the Thaumcraft 2 mod, and is used by its various machines.
There are several different forms of Vis to be found.
Liquid vis is made by putting items into a crucible and is used to power all thaumcraft machines. It can also be made with the Crucible of Eyes, which will damage all mobs near (not you, just friendly and enemy mobs), and use their "stolen health" to generate Vis. Also, Vis (and Taint) Can be drawn out of the atmosphere around, but this is not recomended, as it obviously lowers Vis levels, and raises Taint levels.
'Vis Crystals can be found usually in caves and they are purple crystals that grow on stone. Crystals can be grown using a Crystaline Bell. 'Vis crystals can be made into the ore by placing 9 crystals into a crafting grid as shown below.
Animated Piston can be created using an infuser.
Fusing together a gold ingot, a vaporous crystal and a normal piston creates the Animated Piston. While it cannot be placed in the world like a normal piston would (it is far too skittish for that) it does make a useful component for certain magical devices.