adjective
BAD : If I did the same thing every day, I would be dull.
GOOD : If I did the same thing every day, I would be bored.
DUBIOUS : It was such a dull job that I decided to leave.
GOOD : It was such a boring job that I decided to leave.
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When dull is used to describe a person, it means 'slow to learn or understand': 'He was one of the dullest students I'd ever taught.'
Both dull and boring can mean 'uninteresting' but in this sense dull usually describes a lecture, book, film etc: 'The lecture was so dull that some of the students got up and left.'
bored = tired and uninterested: 'bored students'
boring = causing someone to be tired and uninterested: 'a boring lesson'