REIT structure

please explain to me what are UPREIT and DOWNREIT’s and how can an REIt form a partnership as it itself is a trust?

i imagine REIT as being similar to mutual funds where the AMC is the GP and other contributors are LP and this just doesnt seem to fit in with UPREITS and DOWNREITS.

thanks

http://www.ccim.com/cire-magazine/articles/upreits-and-downreits-gain-popularity