how the pill works

How do birth control pills really work, even during the placebo period?

curiousgirl
asks:
I have been on the pill for over 2 years, and have never missed a pill. I just started my inactive pills on Sunday of this week, and had sex with my boyfriend on Monday. He wore a condom but I am still concerned there is a chance I might still get pregnant. Is it possible for this to happen or am I safe?...