The Hertzsprung Russell Diagram is the "Rosetta Stone" of stellar astronomy and it's simple, very simple. So to put it simply, it's just a graph that plots lots of stars' luminosities against their surface temperatures. As simple as that sounds, it is the key to understanding stellar evolution.
In case you thought "Rosetta Stone" is some sort of educational software for travelers, it is. But it's also an actual stone that Indiana Jones or somebody like that found in Egypt that explains Obamacare in three languages. It sat around in a basement until some anthro-geek was rummaging around and realized we could use it to learn hieroglyphics. Before that no one knew what all those little hieroglyphic symbols meant. Two squiggly lines? A guy with a bird head? What the hell?
The stone dates back to olden times when they didn't have Internet so the king would update his status by having a big stone tablet put in the town square. It was like an iPad but faster. It would be in several languages because they didn't have Google Translate either.
Now back to the luminosity of a star which is its shininess, its rate of energy output, i.e. wattage. The Sun's output is about 4 x 1026 watts or one standard buttload.
Meanwhile the surface temperature of a star and its color are directly tied to each other. Hotter stars are toward the blue end of the spectrum. Cooler stars are red. Other colors are available depending on the model and year.
Stars are grouped into spectral types which can be viewed as either temperature or color groupings. From hottest to coolest the group designations are: O B A F G K M. The order is remembered by astronomy students with the mnemonic "Oh Boy Another Fine Girl (or Guy, or Goat, whatever) Kissed Me."
The majority of stars prefer to lie roughly on a line that stretches from the hot and bright corner to the cool and dim corner. This crowded "Avenue of the Stars" is known as the "main sequence." Mathematically speaking, stars live most of their entire lives on the main sequence and make a comfortable living converting hydrogen into helium. It's good business. Not all of them have the gravitas to pull it off though. Stars of a lesser god wind up tending bar or waiting tables in mathematical Anaheim.
Some stars succeed too well for their own good. Some of the really successful stars let the power go to their head. They eventually cash in all of their assets in one big spree and go supernova. They live fast and die young like James Dean.
Most stars at least get to begin their careers as hot and bright thanks to an energy endowment that comes with their birth certificate. Then they progress to a semi-permanent address somewhere farther down on the main sequence in a reasonably priced neighborhood. What happens later depends on the star's mass. For example, the Sun will eventually move to the upper east side, to Starhaven, an assisted living community for older stars still on the go. It's the home of the world-famous red giants. These stars can no longer peddle hydrogen but they get a decent pension thanks to converting their reserves to Carbon, Nitrogen, and Oxygen and other useful commodities on the exchange.
After the pay-down, when all retirement accounts are exhausted, the stars drop right down into the hospice house of shrunken old white dwarfs. These stars are mostly dead and mostly never heard from again although some become zombies and eat their neighbor's brains. That's if they are in a binary relationship which for stars has always been an acceptable lifestyle choice.