This message means that Google has detected a pattern of unnatural, artificial, deceptive, or manipulative links pointing to your site. Overall site ranking is not affected under this action because Google feels some of the links created are outside your control. Here is a screenshot displaying this sort of penalty:

Why did this happen?

The site was caught buying paid links or engaging in excessive link schemes for manipulating Page Rank that are against Google’s Webmaster Guidelines. Overall site rankings might get affected in future if proper action is not taken. Some negative SEO done to your site can generate this type of penalty.

Suggested Action

Conduct a link audit and get as many artificial links removed as possible, then submit a reconsideration request.

This message means that Google has detected a pattern of unnatural, artificial, deceptive, or manipulative links pointing to your site.

Why did this happen?

The site was caught buying paid links or engaging in excessive link schemes for manipulating Page Rank that are against Google’s Webmaster Guidelines. Overall site rankings might get affected.

Suggested Action

Conduct a link audit and get as many artificial links removed as possible, then submit a reconsideration request.

This message means that Google has detected a pattern of unnatural, artificial, deceptive, or manipulative outbound links from your site. This might affect your entire site or some portions of your site.

Why did this happen?

This happened because Google detected that you have unnaturally linked to other websites that might be related or unrelated to your niche. These links can be paid or exchanged for any other reward. Site wide links, footer links, author bio links, resource pages links, link exchange pages links, paid editorial links, optimized anchor text links that are paid etc. can come under this type of action.

Suggested Action

Scan your site and identify any links that violate Google linking guidelines, such as excessive link exchanges or paid links. Once you have identified those, add a rel="nofollow" attribute to them so that they no longer pass PageRank. You can also use robots.txt to disallow crawling of URL/URLs containing the links.

This message means that Google has detected pages that appear to have been hacked by a third party. Such content is often categorized under cloaking and Google displays a message: “this site may be hacked” in the search results. This is shown in the image below:

Why did this happen?

Due to vulnerable security issues, your site has been hacked by a third party hacker where the existing files on your site were modified. This appears as spam in Google’s index and hence Google reduces the rankings of websites having hacked content on them.

Suggested Action

Login to your Google Webmaster Tools account and check the messages section. Google provides a detailed description of the files on your site which were hacked. To investigate hacking or malware using Webmaster Tools, complete the following steps:

Login to GWT, go to messages.

Navigate to Security Issues :

Sites affected with malware will show a top-level heading of “Malware”, and then categories of malware types, such as “Modified server configuration” or “Error template injection.”

Sites that were hacked to serve spam may display a top-level heading of “Hacked” and hack types, such as “Content injection.”

Sites with a “phishing notification” in Webmaster Tools Message Center may not show any information within Security Issues. By creating phishing pages on your site, the hacker is using your site to obtain users’ login, password, or financial details, often by masquerading as a trustworthy site.

Take the help of a technical expert and get these issues fixed as soon as possible. Also, ensure that your site has adequate security so that it won't be hacked in future.

After you have cleaned everything, request a review of your site.

This message means that Google has detected low-quality pages or shallow pages on your site. Pages having thin content, auto-generated content, scraped content, low-quality guest posts, low-quality user-generated content that was/wasn’t editorially approved might come under this category.

Why did this happen?

Your site has low-quality content pages. These types of content serve little to no value to the users and in order to keep the search results clean, Google takes actions against sites generating such less user-friendly content.

Suggested Action

Conduct a complete content quality review of your site. Check for the presence of thin content or doorway pages on your site. Also, check for several similar content pages that add little value. Get them removed, redirected, edited so that they do not create a negative user experience.

You can also get some help by reading this guide to building high-quality sites.

This message means that Google has detected the presence of automatically generated gibberish, cloaking, scraping content from other websites. In simpler words, your site has violated the Google Webmasters Guidelines.

Why did this happen?

Google considers auto-generated content, scraped content and cloaking as aggressive spam techniques and takes action against sites which apply them.

Suggested Action

Conduct a complete content quality review of your site. Ensure that your site does not use excessive copied/scraped content. Also, ensure that every page on your site serves a specific user purpose. Do not make any page of your website a platform to display advertisements and serve as a bait for the search engines.

This message means that Google has detected user-generated spam on your site. This might affect the entire site or some of the pages on the site.

Why did this happen?

This happened because some or most of the pages on your site have low quality/spammy content found on forum pages, guestbook pages, or in user profiles.

Suggested Action

Check the pages on your site that lack quality content. You might have allowed users free registration and free posting of content through forum profile, blogs or forum posts. This has unknowingly generated a lot of user spam. Identify those pages and get them removed. Then submit a reconsideration request.

This message means that your site is showing different pages to Google bots and users. This might affect your entire site or some portions of your site.

Why did this happen?

Cloaking happens when you redirect users and search engine bots to different pages. It is the practice of presenting different content or URLs to human users and search engines. It is counted as a Black Hat SEO technique and is considered a violation of Google’s Webmaster Guidelines.

Suggested Action

Identify the affected areas on your site and use the fetch as Google tool in GWT to compare the pages as seen by you and as seen by Google. Remember, Google must see the exact page/content as seen by a normal user. Also, check for URLs on your site that redirect users to somewhere other than where they expected to go and fix those.

This message means that some of your pages may contain hidden text or keyword stuffing. This might affect your entire site or some portions of your site.

Why did this happen?

Google has detected some or most of the pages on your site have content filled with keywords. Such content is created solely for the purpose of ranking higher on search engines. Thus, negatively affecting the overall user experience. Also, some or most of the pages on your site might have text that is invisible to the user but visible to search engines in order to manipulate the Google ranking algorithm.

Suggested Action

Identify the affected areas on your site and use the fetch as Google tool in GWT to compare the pages that are visible to Google crawler but aren't visible to a human user. Check for text which has the same color as the page background or hidden CSS styling or keyword stuffed alt tags. Fix these issue and request a reconsideration.

This message means that a significant number of pages on a hosting service are spammy. The entire website is affected by this action.

Why did this happen?

This might have happened because your free hosting site has a lot of user generated content that is mostly spam and serves no real user purpose.

Suggested Action

Get rid of all the spammy user accounts and remove all low-quality content from your site. This is a lengthy task and will require a lot of effort before you can get rid of the entire user generated spam present on your site. Once you are sure all the pages on your site are unique and add value to the end user, you can go ahead and submit a reconsideration request.

This message means that Google has detected some of the rich snippet markup on your pages might be using manipulative techniques.

Why did this happen?

It is possible that your rich snippets contain irrelevant or misleading content or the marked up content is hidden from the users.

Suggested Action

Please go through the rich snippet guidelines mentioned by Google and make sure you follow them. Use the Structure Data Testing Tool after you have made the required edits to ensure there are no errors. This tool can be found under Google Webmaster Tools -> Other Resources section. Any inappropriate markups will be highlighted in red and you may take the intended actions based on the errors. Once the final changes are made and you are sure that the rich snippets are accurate and are not violating any guidelines then you can submit a reconsideration request.