700c is a road bike rim size.
29er is an offroad bike rim size.
But, they are both the same rim size.
The rim is actually 622mm outer diameter.
The tire on an offroad bike brings the outer diameter (of the tire) pretty close to 29 inches.
Same for a road bike – the tire OD is close to 700mm.
Why does any of this matter? Aside from helping explain the likely origins of the 29er wheel size (some hackers adapting road rims to the MTB’s), it is also nice to know you can mount slightly heftier 700c road tires (or even cyclocross?) on your 29er rims if you decide to adapt your bike toward a more on-road or hybrid orientation. This is especially useful if you have an extra set of 29er wheels for your bike- mount some road tires to these wheels and you can quickly change them out for a city ride with your hipster friends. The advice I’ve read is to at least try to get a tire that is still wider than your rim- for instance, use a 38mm wide tire minimum if you have 35mm wide rims.
The next round might be less confusing – the new-ish offroad tire size is being called 650b, just like the road size. I guess 27.5er just didnt have the same ring to it.