The key ingredient of coke or any other similar product is soda, which contains carbon dioxide (CO2). When Coke is purchased in a bottles, the drink is actually under very high pressure to keep the CO2 gas in a stable, liquid state. So, bubbles and fizz cannot be seen inside an unopened Coke bottle. Opening the bottle releases this pressutre and Co2 gas starts escaping, which causes the fizz.
When you shake a can, the carbon dioxide at the top of the can is spread throughout the liquid. Most of the air makes its way to the top of the can quickly, but some become bubbles that can stick to the sides and bottom of the can for a time. These bubbles can't escape when the can is opened, but the pressure is still there. So, instead of a pop of air escaping from the opened can, the pressure is relieved by the release of a burst of liquid.