If you're using rails and have an integer column in database that you want to increment or decrement, rails provides a nice abstraction for you to use. Normally you'd have do something like this:
def increase_credits update_attributes(:credits => credits + 1) end
Then you can call it like:
With the increment and decrement you can do it this way:
@user.decrement(:credits, 2) # decrease credits by 2
It just saves setting up the method and allows you to pass an integer to increment by!