The differences between a Class and ID is with a class one can actually use elements all over the place again, meaning for body copy the same style can be applied all through out. ID's are used for a one time use, meaning when someone goes and uses a certain style on something it can only be used once not twice.

The parent child relationships to CSS is pretty awesome meaning you have the body tag and you put set properties you can make another style and add it in thus branching out from the body, for example this webpage is an example of a Parent Child Relationship going on right now. Everything is linked to one thing but each have their own unique charastic which makes them thir own.

Floats are a weird thing to grasp.... They are able to move boxes around but at the same time DESTROY!!!! EVERYTHING!!!!!!! The web author is able to put together a complete set of boxes and then arrange them in order to how how the floats go, the only thing is math is involved to figure how many pixels someone has to subtract and add.