I found this thread on reddit.

 https://bitcointalk.org/index.php?topic=1349402.0

They explaing what is stake and do mention the forging and harvesting methods, used by NXT and NEM correspondingly.

I couldn't find a lot of clear resources on it and couldn't find anything here on steemit, so I figure I'd bring the issue over to steemit and gelt myself some answers, since I'm couldn't understand the differences among the three. Any help with explanations or external resources is appreciated !