Difficult terrain halves movement. So, for every square that you move over that is difficult, it counts as if you‚ve moved two squares (or three for diagonals). You cannot run through difficult terrain.
Rocks, underbrush, deep snow, steep inclines, rubble, and highly slippery surfaces all count as difficult terrain.
The GM may rule that in certain extreme cases, movement costs in difficult terrain triples (such as a combination of several of the above).