TADAAAA
Image: http://www.atlasoftheuniverse.com/hr.html - Richard Powell; CC BY-SA 2.5
Behold, in all its glory: a Hertzsprung-Russell diagram populated entirely by observational data. Every dot on this diagram is a real star which I think is neat.
Let's talk about the axes first.
From bottom to top, you have luminosity. Brighter stars are higher up than lower stars. Easy! Also from bottom to top, you have "absolute magnitude." I'm pretty sure I've talked about that before, but it's a weird astronomy way of saying brightness. Basically the same thing, I promise.
From left to right, we've got temperature. And color. And also spectral class. All three of these things are so related that you can plot them on the same axis. It's really neat but also kinda confusing, so bear with me. Hotter things (higher temperature) are on the left. Bluer things are also on the left, which makes sense. Hot stuff glows more blue/white than cooler stuff, we see this in candles on Earth all the time! Spectral class is gonna be another space fact, this one is already kinda long - sorry!