The default format in iTunes when importing a CD is AAC and this will be unprotected. It can be changed to other formats such as ALAC, WAV, AIFF and of course MP3, all these formats would be unprotected as well.
An unprotected AAC file will have the file extension .m4a
A protected AAC music track will have the file extension .m4p
It is possible to use iTunes Match to match your own music to tracks from the iTunes Store, this will then allow you to free of charge redownload the same track via your account on other devices. (You of course need an iTunes Match subscription.) See http://www.apple.com/uk/itunes/itunes-match/
As it says on the above Apple page - tracks downloaded via iTunes Match will be in 256Kbps AAC format and will not be protected by DRM. These will therefore once again have the .m4a file extension.
If you purchase a music track from the iTunes Store it is no longer protected by DRM, it will be in 256Kbps AAC format. It will be watermarked with the users AppleID but this is not DRM.
When an AAC file is locally synced via iTunes to an iPhone it will remain as AAC format on the iPhone and will not be protected by DRM.
If you buy an AudioBook via the iTunes Store then as far as I am aware this is still protected by DRM.
An AudioBook will have the file extension .m4b
If you buy a TV show or Movie via the iTunes Store then these are definitely protected by DRM.
If you buy a music video via the iTunes Store then I believe these are still protected by DRM.
Not that it is hugely relevant but contrary to what many people believe AAC format is not a proprietary Apple format, AAC format is an official industry standard.