You have used 'difference' correctly in your opening sentence: What's the difference between (different) and (difference)? How are (different) and (difference) different? What is the difference between a bull and a cow? How are a cow and a bull different? These are all correct but some use difference (difference) as a noun and some use difference (different) as an adjective. Think of it more in terms of where you would put it in a sentence. Sorry I hope this isn't confusing!