A circular argument is one where the conclusion is also one of the premises:
Good websites are the ones that are effectively designed.
In other words, in premise / conclusion form:
If a website is effectively designed, then it is good.
This is essentially defining good to be equivalent of effective, therefore the conclusion is the premise.
Well managed businesses rarely go out of business.
In premise / conclusion form, this is:
If a business is well managed, then it is likely to continue to operate.
Or, stating the implicit premise,
If a business continues top operate AND it operates in a well managed fashion, then it is likely to continue to operate.
Circular argument is also known as circular logic and circular reasoning.