How do you describe anxiety?

Here's a question I've been meaning to ask. How do you describe anxiety to someone who had never experienced it? It runs in my family on my dads side, literally all of his children and himself and his mother have had or currently has an anxiety disorder. My step mom often tells me that she doesn't understand it, what it feels like and to her knowledge it's the feeling of being afraid of everything. She has never had anxiety, not even during the hardest parts of her life so understandable she knows nothing about it.

My little sister (dad and step moms daughter) has been having a hard time in school, not being able to concentrate, hard time taking tests, and generally withdrawn. This has all come about since her parents divorce and she lives with her mom who thought it was ADD that was causing her problems. I suggested it might be anxiety, since she was tested extensively for ADD and was told she doesn't have it. Maybe she doesn't have anxiety, but often times it is, and if you don't know what anxiety is of course you wouldn't consider it.

So tell me, how do you describe anxiety to someone. Maybe you'll give me the words to pass on to her to give her a better understanding.

I describe it as an adrenaline rush, heart pounding, pulse racing, nervousness.