What is the difference between quarterly GDP and annual GDP? I didn't understand how quarterly GDP is calculated. If GDP means Gross Domestic Product in a year, then what does quarterly GDP mean Gross Domestic Product in a quarter? OR it is multiplied by $4$ to estimate for a annual number? I appreciate the help. I tried google but cannot seem to find a page that explains it.
You might not find a page explaining (in an easy manner) how quarterly GDP is calculated because it is done by several different means in different countries and the calculation/estimation methods can get confusing. If you want to know how a certain country generates a quarterly GDP figure, you should check the country's official economic statistics page - the descriptions of the statistic usually list at least something about the collection process.

Usually quarterly GDP is done with a smaller sample and has some estimations done (often based on last year's data). Annual GDP is usually the more accurate measure with a larger sample and more revision time between collection of data and publishing it.