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.
The Stats.OECD web site answers all your questions and offers useful data!
Here is an OECD document explaining the sources and methods used by OECD member countries to compute quarterly national accounts. As explained in the document, most of the OECD countries use 3 types of approach to construct GDP figures: the production approach, the expenditure approach, and the income approach. Among others, those approaches rely on surveys of households and firms.
For the annual figures, the OECD maintains a useful FAQs page, with answers on What is a GDP? How is it calculated? When are the annual national accounts revised? Etc... The source and methods web page is here
Quarterly or annual data for OECD countries can be found here