What is the relationship between inflation and money supply?

Money itself, as it is used in today, is an idea only. When money isn't backed by something solid it is less valuable than previously and causes things to cost more because of the lack of solidity of the money as it is only an idea and everyone is trying to capture the idea and make it concrete. Perhaps also because if there is more money in circulation in theory people have more money to spend, therefore they can afford to spend more.