Chemistry Homework:Explain In Simple English What Resonance Is Please!?

The Wikipedia Definitions just do not clarify enough for me to understand, I guess, because I'm still trying to get an answer here...Is it just a diagram of any molecule with more than two atoms? I think one of the chemistry sites said you do resonance structures when you cannot do Lewis stuctures; When would that be? Can Lewis structures have more the two atoms in it? I thought they could. I just don't understand; Like, I'm trying to picture a situation in which you need to draw resonance structure. From what I saw, they look like the diagrams I draw in Organic Chemistry. Am I right? Do, like, Carbon chains for instance count as resonance structures? What are some examples of compounds who need resonance structures? What IS resonance? I still haven't found an upfront, simple-English, satisfactory definition of the word itself, let alone why the structures are built the way they are.