Professor Einstein’s secretary was so burdened with inquiries as to the meaning of “relativity” that the professor decided to help her out. He told her to answer these inquiries as follows: “When you sit with a nice girl for two hours you think it’s only a minute, but when you sit on a hot stove for a minute you think it’s two hours. That’s relativity.” Learn more at the links below.

Einstein Online
Interactive Experiments In Gravity
Leonard Susskind gives a broad introduction to general relativity (2012)
Einstein’s General Theory of Relativity (2008)
Special Relativity (2008)