Download and examine this table, which contains statistics on various components of the GDP from 2013 to 2015 broken down by quarter. Observe how the United States has gone through growth and contraction at various points during this time period.
Each quarter the BEA produces an estimate of the annual GDP for that year. As the year progresses their estimate of the GDP becomes better and more refined. The fourth quarter estimate is the best estimate, although that number is modified in the following year, as "actual" numbers become available for the previous year.
For fourth quarter of 2014, the GDP is given as "17,615.9". The numbers in this table are expressed in Billions of US Dollars. What it means in words is 17 trillion, 615 billion, 900 million dollars. The US GDP today is heading for 18 trillion dollars.
When you search for this information, you will see slight differences from the numbers reported on various sites to that of the BEA. This slight difference happens because other sites reconcile/modify quarterly estimates to arrive at an annual estimate of GDP, while the BEA just reports quarterly estimates of the annual GDP.