All this data is potentially out of date, and should be taken with a truckload of salt
W-Space stands for "Wormhole Space" are refers to any solar system inside a wormhole. These systems can only be reached by flying through a wormhole and are not otherwise connected to K-Space (aka Known Space) through normal means, such as acceleration gates or jump gates.
For unknown reasons, ships, structures and modules function significantly differently in W-Space than they do in K-Space. For more information on the differences, please see Wormhole Environment Effects.
For more information on wormholes themselves, please see Wormholes.
For information on finding wormholes, please see Exploration Guide.