Girls - Makeup Must-Haves?

I don't mean like specific brands or anything (though that would be helpful) but more in general. Inknow like mascara and eyeliner and lip-gloss... but power, concealer, foundation? I'm not exactly sure what the above do, or which is most important to have. Anything else I'm missing? Also if you have any product you absolutely swear by I'd love to hear it! Thanks a bunch!