Here is a very interesting video of Jon Kabat-Zinn leading a session on mindfulness at Google. Please accept these words from mindfulness.com as an introduction.

"When I take a moment to observe myself having thoughts, I am no longer the thoughts. I get to be and observe at the same time. That's why if I continue to come back to my breath which always occurs in the here and now, it draws me into the present. From that vantage point I can observe as past and future attempt to draw me away from the moment. This paying attention to the here and now, to the breath, to the observing one's thoughts without being critical or judgmental is what many people call Mindfulness."