from “Reasons Why Search Engines Blacklist Your Site” article
…continued here
Cloaking
This is the most effective web marketing spam technique and the most “sinful” because you are deceiving both the internet users and the search engine crawlers. This technique is characterized by serving different web content for the internet users and another different web content for search engine crawlers; the users would see a beautifully designed websites, much appealing to the eyes and the search engines would see the same content differently because what it can only see are well-optimized texts which are keyword-rich.
Invisible Text/Graphics
This is quite similar to cloaking. But the difference resides on the method used. This works wherein the text has a very similar color to the background, making the text invisible to the readers. What shows to the readers are well-written copy but there are actually invisible keyword stuffing that is infused in the page. According to an expert vanguard of ethical SEO, some designers create a “graphic link that is composed of 1 pixel by 1 pixel raster image.” The image is linked to “a hidden inner page such as hidden sitemap.”
Although, there are some internet marketing experts that are actually experts of cheating and makes the web marketing race dirtier than ever. They tend to spam your site by optimizing your site unethically. Yes, they do spend time and effort in optimizing your site the wrong way so that your site gets blacklisted. However, there are some ways how to prevent this thing to happen.