What is spongy mesophyll and palisade mesophyll?

Spongy mesophyll and palisade mesophyll are types of cells involved in the the processes leading up to as well as photosynthesis itself and are located in the leaves of vascular plants.
Monocot leaves usually have 1 type of mesophyll; however, eudicots tend to have 2 types of mesophyll – the spongy and palisade. These cells, as can be deduced, contain chloroplast.
As can be seen in the image below (and without reiterating too much of the information), palisade mesophyll exhibits high levels of photosynthesis and the spongy mesophyll allows for interchange of gases.

(Zachary Hibberd, http://slideplayer.com/slide/2563936/)