How do Improvised Explosive Devices work? I need the details for a story I'm writing.?

I need typical components, how sensitive they are, all that jazz. This might sound like a suspicious question, so I can understand not getting the most detailed answers. I just need enough details to create a ruthless terrorist character that people would fear if he were to really exist. Touchy subject, I know. Just help as much as you can or want to.