Link Optimization should be done only by following the Google Guidelines for Link Building. If we over rule the Guidelines provided by Google or use black hat techniques for link building then we can’t escape from the Google Penguin Algorithm. It was the first released in April 2012, and the last update of the algorithm is in September 2016. Google uses Penguin algorithm to find whether the links in the site is customized efficiently or not.
There are a lot of black hat techniques related to link building like Invisible Link, Link Over Optimization, Web Rings, Link Selling, Buying Link, Link Exchange and more.
For generating links from different size and text of links they will use Invisible links. As the name says, Link will be present in the page, but anchor text will not be displayed, Text size for the anchor text will be Zero. And another common way of generating Invisible Links is by using the same color as of link color as the background color for the web page and generating. If you create links in this way, then Google will take penalty action against your site. And not that all the Invisible links are back, Alt Text for Images can be the invisible link, but it is accepted. Use Invisible Links by following Google Guidelines.
Link Over Optimization:
If you overload your footer will links to other inner pages in an unnatural manner then it links over optimization. And you forcefully place a lot of internal links to pages like “products,” “contact us” then it links over optimization. Because these pages get a lot of traffic as they are. Never try to over do in link building as they are against the link building guidelines.
Multiple sites connected in a circular pattern to get the links are called as web rings. You can be part of web ring, by getting approval from Webmaster. But if you are web ring without webmaster permission then it will have the adverse effect on your site.
Placing some external links for references is a good thing which search engines also love.But if you sell links for passing link juice to another site in unnaturally, then it will surely deindex the site and reduce the traffic to half so never sell links from your site.
There is some websites from where you can get links by paying to them. Never use black hat techniques for links as it may result in the irreversible effect on the site.
If you have two websites and if both and if basic plot of both the site is same it is natural to exchange links between the site, it is accepted to the particular degree. But there also few black hat techniques in which two websites will be in agreement to place external links to each other to get better ranking in search engine.
And there is also the lot of other sites which gives eye capturing services like quality links, links based on niche, and no footprints and so many promises. It is easier to get such links, but at the same time, you are providing the much simpler way for the search engine to take penalty action on your site.
Irrelevant Text and Links:
The anchor text and link to the text should be relevant. If there is irrelevance between the anchor text and page to which it links then the link is of use rather it will result in a decrease the page rank.