The Ides

Can someone tell me where the Ides of March (and all the other months) came from? I know the Romans brought it about and that Caesar was killed on the Ides of March but that they were known before Caesar died (he didn’t bring about the Ides of March with his death). So where did they come from, and why were they used? I know that its the 15th on some months and the 13th on others, whats the deal?

Google works well.