What is meant by major and minor chords?

I was listening to a wonderful radio interview of Paul McCartney, and at one point he was talking about musical techniques employed in writing songs. One that he talked about was shifting between major and minor chords. He gave a few examples, and I could not notice anything going on. What should I have been able to hear? In simple terms, please. I looked at a Wikipedia article on major chords, talking about fifths and triads, which left me clueless.