flat file databases with PHP

So, one more begginers question. I want to ask about "flat file databases". What exactly is it. It's just projects that store their data on simply files, or it's a database system (like Mysql). Are there any rules on how to store and retrieve data from files, so they will be called "flat file databases"? Does anyone know where I can find any reference on the Internet? I'm trying to make small site in PHP (for practice) and use simple files, so I'm wondering if I should first learn about flat file databases and use them, thanks in advance, krizz