Difficult terrain includes the following:
Woods, orchards, growing crops & dense foliage.
Brush, scrub and other clinging vegetation.
Debris, wreckage, loose rocks and boulders.
Areas of ruined buildings.
Fords, streams and shallow water.
Marshes, bogs and thick mud.
Freshly ploughed, flooded and muddy fields.
Sand dunes and areas of deep sand.
Stairs, steps and ladders.
Steep or treacherous slopes (this can include particular hills if both players agree before the game, otherwise hills are considered to be open terrain).
All distances count as double when moving through difficult terrain, so every inch moved across difficult ground counts as 2'. For example, if your unit has Move 4 then it can only move 2" in a wood.
If troops move over open ground and difficult terrain during the same turn, then their movement over open ground is normal and movement over difficult ground counts as double the actual distance travelled. For example, a unit of Men (M4) moves 2" across open ground and then enters a wood. It now has 2" of its move left and this is difficult terrain, so it can only move a further 1" through the wood.