There've been several questions in this group about tuna vs maguro, but such questions were buried in the middle on long threads so I'll try to summarize in a new clean thread.
The Galaxy Nexus family is codenamed "tuna". Using fish names in relationship to devices isn't quite new, since in the past we've had goldfish (emulator), trout (G1), mahimahi (Nexus One), herring (Nexus S) and stingray (Xoom).
There are two actual Galaxy Nexus devices, codenamed maguro and toro. Maguro is the GSM/HSPA+ variant, and Toro is the CDMA/LTE variant. Toro is a bit fatter, hence its name.
In the source tree released yesterday, there are two directories called device/samsung/tuna and device/samsung/maguro. The former contains all the files that are common to both toro and maguro, while the latter contains the maguro-specific files. There's no device/samsung/toro in AOSP yet, that directory will get created when we release the exact code that actually ships on toro.
device/samsung/tuna contains makefiles as if it was an actual device. There's no such device in reality, but the abstraction is convenient for engineering purposes as it allows creating a common system image that works on both toro and maguro, even though it supports neither GSM/HSPA+ nor CDMA/LTE. This is only meant for engineering purposes, and the only real devices are maguro and toro.
Looking a little bit in my crystal ball, we'll be supporting toro in AOSP along with maguro, with similar downloads for proprietary binaries, and the whole process is tied to shipping toro. Be aware that my crystal ball is rarely more accurate than a Magic 8-ball, so you should take this with a grain of salt.
Personally, I prefer working with maguro, as supporting GSM in AOSP is easier than CDMA in my experience.