Best line on the subject: "By their fruits ye shall know them"!
Believe it or not, many Muslims especially in third world countries do not know what the Koran and Hadith say, they learn mostly by oral instruction. Some, like Suffism, believe Islam is a personal spiritual state. They, and many other sec's aren't considered "Muslims" by other more fundamentalist Muslims like the Shia and the worst, Wahabbi's.
Here's a great online book that will teach you more about Islam's written "scripture" and hadith, and how they are inseparable than many Muslim's know.