It's called a concrete encased electrode, and lives at 250.52 (A)(3) in the 2005 NEC.
That's the simple version... no that's the only version
Well, it's not really the only acceptable version.
......"or consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 4 AWG."
From the period 1942-1960, H.G. Ufer tested 24 buildings in Arizona every 2 weeks for resistance values. The installations consisted of 1/2" steel reinforcing bars encased in the bottom of the footings. As a result of these tests, Ufer recommended that a #4 or larger copper conductor be encased in the bottom of the footings, and further testing be performed. The results of the further testing impressed the members of CMP-5, and the concrete encased electrode was accepted.
The term "Ufer Ground" truly only applies to this copper electrode, but has generally been used for any concrete encased electrode.