When do you use em's, px's and pt's?

From what I have observed on SitePoint, it seems that you are typically best off to use em’s when it comes to text, because it takes into consideration the user’s browser default font-size, correct?

When would you want to use px’s or p’s for text?

What do you recommend using on containers (e.g. div’s)?

I have always used px’s because they seem more fixed and quantifiable. But I see some of you usually use em’s on your div’s. Why is that?

Again, why would you want to use one unit over another?