Google Penguin
Google Penguin is a search algorithm update, first launched in 2012, that targets sites manipulating rankings through spammy or unnatural backlinks.
Updated , every quotation and figure read in its source that day.
Penguin is the name of a Google ranking change launched in April 2012 against webspam, link schemes in particular. Since September 2016 it has been part of Google's core algorithm, running continuously instead of in periodic refreshes.
What it targeted in 2012
Google announced it on 24 April 2012 as "an important algorithm change targeted at webspam" that would "decrease rankings for sites that we believe are violating Google's existing quality guidelines", naming techniques "from keyword stuffing to link schemes". It said the change affected "about 3.1% of queries in English to a degree that a regular user might notice", roughly 3% in languages such as German, Chinese and Arabic, and 5% of Polish queries (Google Search Central blog, April 2012). The update became known as Penguin.
Source: Google Search Central blog: Another step to reward high-quality sites (April 2012)
Real time and granular since 2016
On 23 September 2016 Google announced that Penguin, "first launched in 2012", was now part of its core algorithm, with two changes. It became real-time: its data "is refreshed in real time, so changes will be visible much faster, typically taking effect shortly after we recrawl and reindex a page". And it became more granular: "Penguin now devalues spam by adjusting ranking based on spam signals, rather than affecting ranking of the whole site." Google added that it would not comment on future refreshes (Google Search Central blog, September 2016).
What it means for links today
The rules Penguin enforces are Google's spam policies, which define link spam as "the practice of creating links to or from a site primarily for the purpose of manipulating search rankings" (Spam policies for Google web search). Under the 2016 model, links that look manipulative are more likely to be ignored than to sink a whole site. Human review still exists on top of the algorithm: the same policies note that violations can lead to a manual action.
Over-optimized anchor text was Penguin's best-known target; how anchors are judged now is covered in over-optimization and Penguin.
If you think links are holding a site back
Check Search Console's Manual actions report first. Without a manual action, Google says most sites do not need to disavow anything, because it can usually tell which links to trust on its own. With one, remove what you can, disavow the rest, and request a review.