Searing is the art of quickly cooking the outside of a food, usually of a meat. It actually causes a bunch of cook chemical reactions - which I won't go into. It's super easy too, and results in great food.
The goal of searing is not to cook the inside of the food - rather it's to cook and brown the outside. This is because the food either does not need to be cooked through all the way or will be brought up to the appropriate temperature another way.
How to sear (generally speaking):
Heat a pan on medium high to high heat. Using tongs, gently place the food in the pan. It will likely be very smoky/steamy. Turn food, and sear remaining sides.
|Photo courtesy of CookThink.com My iPhone picture antics just weren't cutting it on Searing Day.|
Yup - that's it. Here is a link to what it looks like, as well as some myth debunking.