How to transfer music from iPhone to iTunes library

I noticed my options for syncing my music between my iPhone and iTunes is limited to iTunes purchases. I can transfer purchases from my phone to my library, but what do I do about my music from CDs? How do I transfer my songs and playlists from my iPhone to my libary? I used a different computer to get my songs from the CDs to my iPhone, and I am not about to go hunting down those disks deep in the storage closet in my basement when I have them right here on my iPhone. If I have to transfer the songs as files into a folder to then transfer back into Itunes, what audio format do I use? How can I see what format my existing songs are?