Basically, a rum and coke with a squeeze of lime, but Cuba Libre has much more of a ring about it. Zesty lime and dashes of bitters add layers of complexity...
Yes, the classic recipe calls for just "Rum, Cola and Lime". But a single dash of Angostura bitters adds a much welcome layer of complexity to it if using light rum. Though I don't find it particularly necessary if using Havana Club Especial or Bacardi 4 aƱos for example.