What is it with vitimin A,B,C etc

I don't understand it what are they good for like what's vitimin A good for or B C and the rest?

Vitamins allow you to rationalize that somehow those little colored pills will go down in your stomach and change all that junk food into spinach, broccoli and cauliflower. :-)

Harvard Public Health:
http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/vitamins/