"Don't worry" is what you would say to calm someone down or reassure them. You can also add a "about it" at the end to make "don't worry about it" which signifies "you're welcome" or forgives an apology. E.g Mary, don't worry about the mess. I'll clean it up. (using it in this sentence as a reassurance) E.g Linda: Sorry for forgetting your birthday Mark. Mark: Don't worry about it Linda. I forgive you. "No worries" is usually used as an alternative to "you're welcome" (after someone says thank you). It's also used to respond to someone who has said "sorry" (instead of "it's ok"). E.g Linda: Thank you for the beautiful flowers Mark! Mark: No worries Linda. I'm glad you like them! Linda: Sorry for crashing your car Mark! Mark: No worries Linda. I was going to buy a new one anyway.

'Don't worry' : This is what you say when you are genuinely telling someone not to worry about something. If a person is upset, concerned or anxious about something, you can say 'Don't worry'. 'No worries': This is a formulaic response to 'Thank you', used mainly by Australians. It has the same function as 'You're welcome.'