There really isn't an interface. Mint uses your login to get the state of your accounts but does not allow you to transfer money or anything else, it is read only. Then it tried to catagorize all the transactions you have based off their database of companies. You can track your spending over time in different categories. It sounds like nothing but unifying all your accounts in one place is very useful.
Is anyone here able to describe how their interface to your bank account works in more detail? I haven't heard a great explanation of it.
Part of me is still amazed that many people have happily typed in their banking passwords.