What do the different ski slope colors mean?

1) Green (usually represented by a circle): these are beginner slopes and are not very steep. A New skier should be able to navigate these without losing control or catching too much speed, but more advanced skiers tend to find these too flat. 
2) Blue (usually represented by a square): These are intermediate slopes with some steeper or more narrow runs. Skiers of all skill levels can enjoy these, but beginners might need to take it slow on some of the steeper parts.
3) Black (usually represented by a diamond): These are for advanced skiers only and have steep drops and/or obstacles. 
4) Double Black (usually represented by two diamonds): These are only for the most experienced and advanced skiers and usually include extreme drops with significant obstacles.