What is a Google Penalty?

In a nutshell, a Google penalty is assessed when a website is not in compliance with Google’s quality guidelines and the website receives reduced or no rankings in search.

Google penalizes websites that violate Google's webmaster quality guidelines. There are two types of occurrences commonly referred to as “penalties”:

1. Manual actions

2. Algorithm filtering

Manual actions:

Google issues a manual action against an individual website when a human reviewer at Google has determined that pages on the site are not compliant with Google's webmaster quality guidelines. Most manual actions address attempts to manipulate search result rankings. If a site is affected by a manual action, Google will note it in the Manual Actions report in the Search Console.

List of manual actions:

• Site abused with third-party spam

• User-generated spam

• Spammy free host

• Structured data issue

• Unnatural links to your site

• Unnatural links from your site

• Thin content with little or no added value

• Cloaking and/or sneaky redirects

• Pure spam

• Cloaked images

• Hidden text and/or keyword stuffing

• AMP content mismatch

• Sneaky mobile redirects

• News and Discover policy violations

Algorithmic filtering:

Algorithmic filtering, on the other hand, is a fully automated part of Google’s ranking algorithm. Google can detect what they consider to be manipulations and “filter” this site accordingly. Information about imposing this filter from Google doesn’t appear in Google Webmaster Tools.

Additional information:

https://support.google.com/webmasters/answer/9044175?hl=en