Elves are a exception to this general rule for forests. Not only do they possess full movement in forests, but they also gain considerable defensive bonus. Dwarves are another exception to this rule; though they are able to plow through the forest without much loss of speed, their utter unfamiliarity with the terrain causes them to receive no defensive bonus.
Most units have 50% defense in forests, but cavalry are limited to 30%. Elves, on the other hand, enjoy 60 to 70% defense, even their mounted units. Dwarves generally receive only 30% defense in forests.