1: This section with the '=' symbols marks a stockpile. It's where dwarves store various crafts, barrels (not pictured...denoted by a % with a yellow background), and food.
2: This symbol represents a cage for trapping creatures. You'll see these a lot.
3: This is a pile of rocks which come from digging away the mountain. You'll see them all over. They can be crafted into all manner of useful items.
4: The matrix looking green crap is the ground. On the inside, to the right of box 10, you'll see the symbols are white, instead. Smoothed stone floors in the fort are represented by + symbols. Some people use a variegated tileset, which results in what you see here. Others will have ground that displays as all periods. You eventually get used to both and can easily pick out what is where on any map.
5: Dwarves are shown as these guys here, or as smiley faces. Different colors indicate different professions. These guys are all military recruits, charging off stupidly to their deaths.
6: The E is an elephant. They murder dwarves in wonderful fashion. Get used to them. Above and below him, the brackets are discarded armor or clothing. The grey blocks to his right represent exposed walls of un-mined rock. The blocks with the cross on them represent stone doors. The small 2's that fill the room behind the two doors are the bones of animals or dwarves.
7: The ^ symbol denotes a trap. The parallel lines below the traps represent a smoothed wall. Below these lines, you see the grey wall of the bone room, which is not smoothed. Different colored symbols in grey rock walls just represent types of exposed mineral, gems, or ore veins.
8: This is a pile of dead dwarves, an Elephant, and a cloud of Miasma. Those are the three most prevalent features in Boatmurdered. Miasma is the purple blotch that shows up when corpses begin to rot. It makes your dwarves angry, which usually leads to hilarity.
9: The two green asterisks are gems. These have been mined out of a wall, already; and are awaiting storage.
10: These are siege engines. In this case, two ballistas aimed at the hallway with all the dead dwarves.
11: The black spaces with symbols like these are areas of the mountain that have yet to be mined.