What causes people to faint?

Okay, so I was in science class, and I was watching a video on the whiteboard. All of a sudden I began to feel really hot, and a little queasy. Then about ten minutes later it went really bad, I felt really sick and the people who i was sitting next to told me I looked really white in the face. I stood up and everything went yellow and I didn't really know what I was doing. So a teacher sent me to stand outside and get some air, I got halfway down the corridor and I fainted. All I remember is waking up with a teacher and one of my best friends by my side. My question is, what caused me to faint? I was just watching a video at the time..?