Times goes slowly when something is close to gravitational sources (like planets, suns, or even black hole) .
http://en.wikipedia.org/wiki/Gravitational_time_dilation
Also times goes slowly when something is moving fast. This is why time goes slower to the people that are on the space (they are moving relatively fast). And also why astronauts are considered the first time travelers (to the future), even if only for a fraction of a second.
http://en.wikipedia.org/wiki/Time_dilation
Finally, it seems that scientists did not reach a consensus about traveling to the past yet, but there are several hyposesis about this in discussion ritgh now. For instance, some scients believe that even if we cannot go to the past, at least we can send a message to the past (given that in the past, some one has a machne able to recive the message).
