A penalty constitutes a downgrade of web documents in the search engine rankings or even the exclusion of a webpage from the Google index. You can test as to whether a domain is affected by a penalty by typing the domain name in the Google search box. If no results are visible or results show up in very low positions, it may be an indication of a penalty.
The Google search engine assesses web documents with an algorithm that evaluates the pages based on their relevance and quality. Moreover, it is checked as part of the assessment whether a document violates the Google Webmaster Guidelines. If there is a clear violation (such as keyword stuffing) the webpage may be penalized by the search engine.
There are different types of penalties imposed on websites. There are generally three types of penalties:
The minus 30 penalty is the downgrading of a domain on the rankings in the Google index to position 31 or lower. The results are being placed on page 3 onward in the search results. This type of penalty is usually applied to the entire domain.
A minus 950 penalty means that the domain can no longer be found among the top 100 search results, however, is not completely banned from the ranking. This type of penalty often relates to individual URLs or keywords.
Index exclusion exists when the page is no longer available on the Google index. This can be checked by means of a site search.
The difficulty in determining if it is a penalty (especially in the case of a minus 30 penalty) is to distinguish whether the website is no longer ranking well because of a lack in popularity or whether a breach of the guidelines exists.
If you are in compliance with Google’s guidelines, penalties can usually be avoided. If relevant content is available to the users of your website and you do not get involved in unauthorized SEO techniques such as doorway pages, cloaking, or hidden text, it is unlikely you will get penalized. You should pay particular attention to external links, because unnatural link structures may be penalized by Google.
Should it happen that your domain has been subjected to a penalty, Google offers the affected webmaster an opportunity to actively communicate about it. As part of a reconsideration request in the Google Search Console, you may file a reinclusion request. You can inform the search engine that the violations against the guidelines have been corrected and ask for the reinclusion of the domain in the index.
It is recommended to first ascertain whether there is indeed a penalty, or whether the ranking changes are transitory.