Understanding Google algorithm updates helps you design your website so it indexes/ranks well on Google and other top search engines.

Google continually changes/updates its SEO algorithm to improve search results for Google's audience.

(SEO Algorithm is a fancy word for computer formula determining indexing/ranking in the natural or organic or free search engine results).

Hundreds of Google algorithm updates are minor, here is a list of the major updates/changes that affect search results.

2021 Google algorithm updates:

Page Experience Update — June 25, 2021
Google confirmed that the new Page Experience update has started to take place. The update will be complete by the end of August 2021.

Broad Core Algorithm Update — June 2, 2021
Danny Sullivan of Google made it known that a broad core update is about to released plus related updates will take place in July.

Product Reviews Update — April 8, 2021
Sites that provide in-depth product reviews versus light minimal content created simply to draw clicks will benefit widely from this update.

Passage Indexing — February 10, 2021
Google released the "passage indexing" udpate said to effect US/English queries which they estimated would be approximately 7% of queries.

2020 Google algorithm updates:

Google Core Update — December 3rd, 2020
Google announced a core update and later followed that up with an unnamed update on December 17, 2020.

Featured Snippet De-duping — January 22, 2020
Google announces URLs in Featured Snippets will not appear in traditional organic results too. This has significant implications for rank-tracking and organic CTR.

2019 Google algorithm updates:

BERT Update — October 22, 2019
Google implements the BERT natural language processing (NLP) model. BERT helps interpret natural language searches and understand context. BERT stands for Bidirectional Encoder Representations from Transformers.

Global Core Update — March 9, 2019
Google confirmed that a global core update had taken place and that it was the 3rd of it's kind since they began describing updates in this way. Some felt this update was one of Google's biggest in recent years. Google itself announced the update as the "March 2019 Core Update" rather than leave it to the SEO community to name it which often has been the case.

2018 Google algorithm updates:

"Medic" Core Update — August 1, 2018
Google verified a "broad core algorithm update," with reports of major impact. Large scale impact was seen in all markets. Google’s Danny Sullivan recommended following the advice provided in the this post by SearchEngine Journal March 9, 2018 update.

Chrome Security Warnings — July 24, 2018
Google began by warning visitors of unsecured forms recently and now as of Chrome 68, all non-HTTPS sites will be marked as not secure. Although the changes took place on July 24th, only users that have installed the latest version of Chrome will be alerted.

Mobile Speed Update — July 9, 2018
Mobile page speed is now a ranking factor when searching on mobile devices. This change supposedly is only going to affect slower mobile web sites.

Snippet Length Adjustment — May 13, 2018
Google returned, after a short test, to most snippets limit length of 150-160 characters.

Mobile-First Index Roll-out — March 26, 2018
Mobile friendly pages are important and it should be clear to all that the Internet is headed to a mobile-first mindset since Google has announced that they are rolling out this update.

Broad Core Algorithim Update — March 9, 2018
In an attempt to benefit under-rewarded pages, Google released the Broad Core Algorithim Update. Google did not provide much in the way of details only advising everyone to "continue building great content."

2017 Google algorithm updates:

Fred — March 7, 2017
This update punished low-quality content. Google confirmed this update on March 24th but did not share any more details other than all the answers about Fred could be found in Google's Webmaster Guidelines.

Chrome HTTPS Warnings — October 17, 2017
Google made it clear with this update that sites with secure pages (https vs http) would have an upper hand in search engine results. This should remove any doubt whether it's worth the effort to make your site secure.

2016 Google algorithm updates:

Penguin 4.0, Phase 2 — Oct 6th, 2016
Penguin 4.0, Phase 1 was rolled out on Sept 27th, 2016.   Penguin algorithm update devalues bad links.  The concept is that quality inbound links tying your website to other relevant portals on the internet increase the relevancy of your content.  In concept, other websites will not link to your website unless your content is applicable or provides value to their audience.  When Google first started Page Rank was largely consisted of the number of links pointing to your website - Larry Page noticed a correlation between quantity of in-bound links to relevancy of quality content.  However, many SEO specialist that knew the quantity of inbound links had a directly correlation to ranking - started gaming the system and started building links whether they where quality or not (link farms, or bad links, will now potentially get you banned from top search engines).

Possum — Sept 1, 2016
  was to diversify the local results and prevent spam from ranking well.  In the past if a business was not located in a physical city being searched it did not rank well, even if it was just outside the city limits.

Mobile-friendly 2 — May 12, 2016
(Mobile Update "Mobilegeddon" was on April 22, 2015 - although immediate effects where not as significant as expected).  If your website is not mobile friendly it will not rank well on mobile or responsive devices such as tablets or smaller computer screens.  I believe this Google algorithm update effects indexing/ranking on all searches - but this is just Jordan's opinion.

Here is a link to Google's mobile-friendly test, via Think with Google: https://www.google.com/#q=mobile+friendly+test  (You should run your website through this Google tool.  They will also send you a report detailing helpful suggestions to improve your site's rendering and performance).

AdWords Shake-Up — Feb 23, 2016
Google removed the right-column sponsored ads, and put these ads as the top 4 positions and at the bottom of the page.   This resulted in leaving only up to 10 natural organic positions on the 1st page of search results.

2015 Google algorithm updates:

RankBrain — Oct 26, 2015
(could have been earlier in the year).  RankBrain made it possible for Google to pull up search results for content relevant to the keyword/phrase entered into Google's search box even if the keyword/phrase was not in the content.  In short, it helped contextual based searches.   There was buzz about SEO not being keyword/phrase based anymore - but that is simply not the case... you should still build your website architecture around your top keywords/phrases.

Panda 4.2 — July 17, 2015
(this was technically a "refresh" not a new update).  Panda is designed to push down lower quality or sites with "thin" content ... meaning content that had little or no added value.  It also helped small and medium size sites that had quality content rise in rankings.

The Quality Update — May 3, 2015
Google acknowledged a core algorithm change impacting "quality signals."  Here are a few links that share Google's "quality" views: Google's 1.4 Create Valuable Content Course. Here is more Google guidance on building high-quality sites (it's from 2011 but still good stuff): Google Webmaster Central Blog Post

2014 Google algorithm updates:

Pigeon Expands (UK, CA, AU) — Dec 22, 2014
(Original update hit USA in July 2014).  Pigeon was designed to provide more useful local search results.  Google stated this local search algorithm ties deeper into their web search capabilities, including hundreds of ranking signals used in search along with search features such as Knowledge Graph, spelling correction, synonyms, and more.

Pirate 2.0 — Oct 21, 2014
(this is another update - not the original launch). Pirate is designed to combat software and digital media piracy. In short, don't pirate or plagiarize - quality original content is the most heavily weighted criteria on many top search engines.

"In the News" — Oct 7 2014
Google expanded news links to a larger set of potential sites.  Major news sites reported significant traffic changes.

HTTPS/SSL Update — Aug 6, 2014
Google announced they would give preference to secure sites, adding encryption would provide a small rankings benefit.  They stressed this boost would start out small but implied it might increase over time.

Payday 3.0 — June 12, 2014
Targets spammy queries.

Page Layout # 3 — Feb 6, 2014
Also known as "Top Heavy" - it penalizes sites with too many ads above the fold.

2013 Google algorithm updates:

Hummingbird — Aug, 20, 2013
Been compared to Caffeine - it was a major update still relevant today.  It has  over 200 criteria ... designed to return better results.   Here is a "Periodic Table of SEO Success Factors" that detail much of Hummingbird (and SEO in general):

 

I could go on regarding additional Google algorithm updates - but there is plenty of information available above.  A few other updates you should research are: Domain Crowding, Phantom, and here is an old "Google Inside Search" video from Amit Singhai - very nice: Search Quality