If I'm not mistaken ( I'm not really experienced..) composition is about placing everything in a certain balance that it will look nice to each other. But on the other side, I could be completely wrong. xD
composition is how you organize the elements in your signature. i.e lighting effect placement etc etc.
It can also refer to the sizing you choose. Some stuff looks better on smaller sizes and some look good in larger ones.
not quite,but they are very similar. flow is how your tag fits together, while composition is how you put it together.
Yup, like people said, it's how thngs take place, regarding each other. For example, if your stock or render has some lighting on it, it's important to keep that in mind when placing your light source. So as said, it's very similar to flow actually.