When we say we are worried about something, we might mean that we are concerned about it. But worrying as an anxiety disorder is something more complicated, it is the circular process of invasive or obsessive thoughts that can't be controlled. So I might say I am worrying about my exam results, which might mean I am concerned, or if I really am worrying about them then I cannot think about anything else and am imagining all the catastrophes that are coming – I'm going to fail, I'll be thrown out of school, I'll never get a job, my life will be one long mess.
For some people, worry starts out as being a form of planning for every eventuality. Everything must be considered and anything that could go wrong needs to have a solution. Unfortunately, the sufferer experiences the anxiety that goes with the situation as if it were real. If either this could happen or that could happen, the worrier experiences the negative emotion (usually anxiety) from both of them, each and every time they think about them. Compulsive planners might be reluctant to give up their planning.
Worrying is often accompanied by insomnia.
Worrying is very treatable; you can learn techniques for not engaging with the anxiety-provoking thoughts.
I wrote my dissertation at Goldsmiths on worrying.