Green coffee beans taste grassy and raw. Roasting is what turns them into the coffee we know, and how far a roaster takes them changes the cup dramatically.
Light roasts
Light roasts stop soon after the beans begin to crack audibly in the roaster. They keep more of the bean's original character, so you will often taste bright acidity and fruity or floral notes that reflect where the coffee was grown. They suit pour-over and other filter methods.
Medium roasts
Medium roasts balance origin flavor with the caramel and nutty sweetness that develops with more time in the roaster. They are an easy everyday choice and work well across drip, pour-over and espresso.
Dark roasts
Dark roasts are taken further, often until the surface turns oily. Roast flavors take over: chocolate, smoke and a fuller, heavier body, with lower perceived acidity. They stand up well to milk.
Does roast change caffeine?
Not much. The differences in caffeine between roast levels are small. Beans lose mass as they roast, so a scoop of dark roast weighs less than a scoop of light, which affects how much coffee ends up in your cup more than the roast itself.
Freshness matters most
Whatever the roast, look for a roast date on the bag. Many people find whole beans taste best within a few weeks of roasting. Store them airtight, away from heat and light, and grind just before brewing.