WARNING: Blackhat SEO are things NOT to do to improve your website rankings. They can result in lower search rankings or get your site completely banned from search engines.
It use to be that people could perform these blackhat SEO tactics on a website and increase their search rankings dramatically. Some developers knew that it was wrong to do these things and still did them anyways, and others did them without knowing what they were doing was wrong. Now days search engine robots have gotten much better at recognizing these tactics and will penalize your site for doing so.
I wanted to explain each tactic just so you are aware of what NOT to do.
Doorway or portal pages are created to improve their rankings in search engines for particular keywords/phrases. These are pages that are specifically designed for search engines only. The idea it to make search engines think that users are seeing one page, when they are actually seeing another.
Blackhat cloaking is a way to serve up a doorway page. It is when a website displays two different sets of content, one for the search engines and another for the user. The idea is to try and trick the search engines by giving them content that is strictly to move up in search rankings. The user however will see something completely different that is more user friendly and easy to read. Based on your IP address, they are able to tell if you a search bot or a actual human.
Learn more about Cloaking and Doorway Pages
There are a number of ways you can display duplicate content. One way is to copy and paste their content onto tens or even hundreds of pages within their site, specifically to add more pages and content to their site. They could also create a totally new website or microsite that contains the exact content as the original site. People do this to add inbound links to their site as well.
Learn more about Duplicate Content
Hidden Text & Links
The idea here is to add hidden links and text that only the search engines can see. Usually a developer will make the text the same color as the background so it is not visible to a user. The text or links usually have keywords or phrases repeating multiple times, in order to try and fool the search engines into thinking your page is more relevant for those keywords.
Learn more about Hidden Text & Links
Keyword stuffing is exactly what it sounds like. You are trying to stuff as many keywords or phrases onto a page as possible in order to rank higher in search results. Normally developers will hide this text so it is not visible to users.
Learn more about Keyword Stuffing