Easiest way to browse iPhone filesystem
Despite the fact Steve Jobs didn't support this idea because it's making things complicated for the users, there are some apps which will allow you to do so.

There is i-FunBox which allows you to browse through your iPhone files on your Mac. And if you're a Windows user, you've more options. There is iPhoneBrowser or iPhone Folders.
All of these solutions will allow you to browse the iPhone file systems using your computer. 

If you want to be able to browse on your iPhone yourself, I'm afraid I only know one solution, called iFile. But this is only accessible on a jailbroken device...