Git manages tags/refs, pointing to the commit-objects, organized in a DAG-graph of trees, consisting of the smaller-trees and file-blobs objects (sometimes moved to a packs)

Articles
http://jaxenter.com/git-is-simple-on-the-inside-49426.html
http://wildlyinaccurate.com/a-hackers-guide-to-git
https://habrahabr.ru/post/313890/

Tips
https://medium.com/@porteneuve/30-git-cli-options-you-should-know-about-15423e8771df
https://github.com/ninrod/tricks/blob/master/git.md

Presentations
http://talks.rosalab.com/20140823-05
https://speakerdeck.com/matthewmccullough/git-graphs-hashes-and-compression-oh-my-1 (http://youtu.be/ig5E8CcdM9g)
http://www.slideshare.net/chacon/git-101-presentation
http://www.slideshare.net/chacon/getting-git
  Slide 78 - Object database format

http://www.infoq.com/presentations/A-Tale-of-Three-Trees
  working-dir, index/staging, repo/head

Smith - Knowledge is Power
http://www.slideshare.net/tarkasteve/knowledge-is-power-getting-out-of-trouble-by-understanding-git
https://www.youtube.com/watch?v=sevc6668cQ0

Ceatsheets:
http://zeroturnaround.com/rebellabs/git-commands-and-best-practices-cheat-sheet/

Visualizations:
http://onlywei.github.io/explain-git-with-d3/
https://github.com/onlywei/explain-git-with-d3

Tutorials
Kong - Git Tutorial
http://www.cs.cmu.edu/~soonhok/talks/20140519_Git.pdf

man:
git everyday