if you want to run a sale, or someone says - i really want that, but i'm cheap and i want a discount, you give them a code. however the amount off only covers your side. and telling them that 50% is not really 50% is confusing.
you set the amount you want off in dollars or percent, how many times it can be used (each time covers any number of things ordered). how long its good for and i think one other thing. give the code, they use it at check out.
in general i don't use them unless i have to.
---Mike Savad
MikeSavad.com