Eastern Standard Time is 5 hours behind Greenwich Mean Time. For example, if it were 5:00pm GMT, it would be 12:00pm EST. It is also 3 hours ahead of Pacific Standard Time, which means if it were 3:00pm EST, it would be 12:00pm PST (where Los Angeles is located).
The entire East Coast of the United States, including Washington, DC and New York City, is located in Eastern Standard Time.