The iPhone uses its own .m4r format for ringtones. This format is the same as Apple's AAC music format, which uses the .m4a extension. You can literally take an .m4a file and rename the file extension as .m4r to create an iPhone ringtone! Make sure the file you're renaming is under 30 seconds in length or it may not work.
Ringtones can also work as alarm clock alerts on the iPhone, but not the iPod touch or iPad. In addition, the iPhone does not yet allow you to create custom SMS tones or e-mail alerts. Hopefully, that capability will be included in a future release of iOS.