What a great inquiry! I love getting questions about the science behind things you might see everyday. So here is your answer:

Soda is made of carbon dioxide gas (like the gas that you exhale) dissolved in water. The way to get carbon dioxide gas to dissolve is by placing it under a higher pressure than the pressure outside of the bottle. By shaking up the bottle you cause the gas to escape the liquid (you know this is happening because you see bubbles going to the surface). When you open the bottle the high pressure inside escapes to the lower pressure outside. This gives as a result equal pressures both inside and outside the bottle. The faster you open the cap the faster the pressure can escape usually resulting in your soda to 'explode and make a big mess everywhere! To avoid this, you can open the cap very slowly allowing the pressure to escape slower.