what are retrofocus lenses?

Can someone explain it in basic terms. I did a googled search and got confused because different websites say different things.

I sometimes hear some Zeiss and old Canon lenses refered to as that and it got me wondering.

I recently got one of said lenses and it is huge for what it is and was wondering if that was the reason.

The way I understand it is that retrofocus lenses are lenses with an element to expand and then converge light in order to create enough space to clear the mirror.

The pic in this thread explains it quite clearly

http://www.dpreview.co​m/forums/post/42536453

That's what confused me, so would that mean every DSLR wide angle lens is a retrofocus?

In a simple lens, the distance from center of lens to focal plane is the same as FL. In a complex lens of ordinary (non-retrofocus, non-telephoto) design, the Back Focus distance is about its FL. In a 'ordinary' complex lens, the rear node of a 17mm lens might be 17mm from the focal plane.

In a retrofocus lens, the rear node is farther from the focal plane than its FL. In a retrofocus lens which is 17mm FL, the rear node might be 45mm from the focal plane., the rear node is forward of where it should be by its FL.

Contrast that with a 'telephoto' lens, which might have FL of 500mm (19"), yet the entire (canon) lens is only 15" long! Its Back Focus distance is less than its FL, its rear node is behind where it would ordinarily be for its FL

And, yes, every WA lens for a dSLR is of retrofocus design, so the reflex mirror can be accomodated.
Hassy had a 40mm lens for its SLR which was retrofocus, but it also had a non-retrofocus design 40mm lens that required a special body which used an external viewfinder rather than a reflex viewfinder.