Medium format refers to a camera which uses a larger format film than the standard 35 mm (and dslrs). Hasselblad and Mamiya are examples of these types cameras.
"Generally, the term applies to film and cameras used to produce images larger than the 24 by 36 mm of 135 film, but smaller than the 4"×5" size, which is considered to be large format."
http://en.wikipedia.org/wiki/Medium_format_(film)
As for the D700, 900 etc, they have a full frame sensor. What this means is that in the standard non-professional cameras such as the D80 or the Canon XTI, they use a smaller sensor (what records the image) as compared to the standard 35mm film frame whereas the D700 records an image the same size as the 35mm film frame. If that isnt clear, heres a sight to explain it
http://www.the-digital-picture.com/Canon-Lenses/Field-of-view-crop-factor.aspx