Light green is just grass, dark green is light undergrowth (double movement cost, 20% concealment). You can enter squares that contain small trees, and doing so gives you a +2 cover bonus to AC (it assumes you are placing the tree between you and an attacker) - note some of the creatures are already in squares with small trees.
The shaded area at the root of the fallen tree is where it was uprooted; travel through those dark squares is extremely hampered, and each square counts as four; the root bulb acts as a wall.
The branches and foliage at the top of the fallen tree will cost you 4 squares to enter each square as well, and grants 30% concealment.
The trunk of the fallen tree is acting as a low wall, granting standard cover.
The broad shaded line that follows the north side of the road is an escarpment leading up from the road; each such square also costs 4 squares to enter.
The blob of orange is a raging fire.
The gray cloud of fog is ~30 feet high; the light outer ring is above ground level, so doesn't count as obscured unless you are flying.
Remember, you cannot take 5-foot steps when your movement is hampered.