Difficult terrain includes me following:
Brush, scrub and other clinging vegetation.
Debris, wreckage, loose rocks and boulders.
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.
Woods, orchards, growing crops and dense foliage.
Troops cross difficult ground at half speed, so 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 at full speed and movement over difficult ground is reduced to half speed as normal. For example, a Man moves 2" across open ground and then enters a wood. He now has 2" of his move left. This is covered at half speed as it is difficult terrain, so be can only move a further 1" through the wood.