What is the difference between use and abuse of substances?

I thought it was the same thing

Think of it this way: you can take (use) a prescribed medication but when you take too much of it, take incorrectly, or take a prescribed medication belonging to someone else, that's abuse.

This also generally applies to illegal drugs.

For clarification see my source.

Use is when something is used in therapeutic doses for therapeutic reasons. For example, marijuana smoking is used to help chemotherapy patients combat nausea. Now, if that same chemotherapy patient continued using marijuana even when he was not nauseous, that would be abuse.

Another example is if one is taking pain medication for pain, then continues taking it because of the euphoric feeling it induces, rather than to treat pain.

USE
In WWII many soldiers were given morphine to kill the pain. They suffered less and received treatment. This is the correct way to use morphine.

ABUSE
Unfortunately, some soldiers became addicted to it and left the war as drug addicts. This is an abuse of an otherwise very beneficial medicinal substance.

use of substances means proper n judicial usage of substances...
abuse of substances may refer to over-use or over-exploitation of substances