Coffee is brewed from the roasted seeds ("beans") of the coffee cherry. The two species that matter commercially are Arabica (sweeter, more acidic) and Robusta (harsher, more caffeine).
Roast level is the big lever on flavour: lighter roasts keep more of the bean's origin character, darker roasts taste more of the roast itself.
Shares its stimulant, caffeine, with Tea — though a cup of coffee usually carries roughly twice as much.