I came across this stuff a few years ago and I kind of knew it all but without ever thinking about pulling it together, so having a reference point since then has been really useful. And I really like this article as it goes through these fundamental rules clearly and succinctly and picks some nice examples to illustrate.
Author of the blog post, Carolann Bonner says:
Gestalt is a term used in psychology which expresses the idea that the whole of something is more important to our understanding than the individual parts. The Gestalt principles describe the way our mind interprets visual elements.
The principles she then goes on to cover are: