One of four trademarks used by the Coca-Cola Company to identify their main product which is a sugary soft drink flavoured with cola nut and coca leaf extracts amongst other things. The other three marks that refer to the same product are the shape of a Coke bottle, the "dynamic ribbon device contour" and the word "Coca-Cola" which should always be hyphenated.
Coke is a brand name (that's why Pepsi - or any other cola drink isn't Coke. However order Coke in a bar and they'll often serve you any old rubbish in return for your cash.
Get me a rum and Coke