Earlier quoted context omitted.
It's a mess when you learn there are also 6in4, 6rd and teredo...
Out of all of those, 6rd is actually a success, though 6rd is built on top of 6to4. Funnily enough, the first ISP to apply 6rd has actually inverted its network and is now using 4rd to provide IPv4 to some of its customers. Not just that, but also 4in6 (IPv4 in modern networks) and 4over6 (an extension to DS-Lite), and don't forget 6in4 being called SIT in an early draft, followed by SIIT being used in another protoc…
6in4 is just a basic "put the v6 packet directly into the payload of a v4 packet" tunnelling approach. It's a sensible way of tunnelling one protocol over another (and it's the same thing as 4in6, just the other way around).
HE's tunnels are 6in4, and 6to4 and 6rd are automatically-configured 6in4 tunnels.