Worms are parasites (organisms that live inside your dog).
If you're worried that your dog might have worms, here are some of the signs to look out for:
(Note that your dog might appear perfectly fine, only appearing unwell when already heavily infected)
Weight loss, especially if they also seem very hungry
Some dogs, especially puppies, may have a round or swollen abdomen
These signs may also indicate a different problem, such as bacterial or a viral infections or having eaten something they shouldn't (like a toy or a stone). Vomiting and diarrhoea are common signs of lots of different diseases, that's why it's important to see a vet.