Google Penguin Update

Google Penguin updates are primarily focused on finding out which sites use spammy links to game google system and removing them from search results.

These updates were rolled out in April 2012

This update is not focused on user experience but on combating link spam.

You would want to note that penguin does not care about how good the content on a site is.

If it finds that the site is trying to game the system it will devalue the links and may even penalize the site. So Google is willing to punish even sites with great content if they use spam to try to get to the top

Google Hummingbird  

Hummingbird is not an update. It is not designed to combat link spam or low-quality content.

Humming is a redesigned version of the entire algorithm.

It is believed to have been developing as more and more people now use mobile devices to use google. The website is not penalized by hummingbirds.

It came on September 26, 2013,

RankBrain 

RankBrain is a machine learning system that google use to filter result and select relevant pages based on what user searching for.

RankBrain is designed to understand speech and give results that closely match what the user is looking for.

• The rank brain uses AI.
• The rank brain learns and becomes better –all by itself.
• A site may be deindexed :
• A manual penalty can be levied for several reasons.
• Legal reasons: 
  

Content can also be deindexed for legal reasons. If a court order googles to remove a certain result, it will have to comply.

Malware :

And if Google finds a certain site is infected with a malware like a virus or a Trojan, it will be removed from the index.

Spam report:

A sites competitor can also file a spam report against a site they suspect is using black hat methods.