There are a number of ways to get penalized by search engine websites like Google or Bing. The way these sites work is by indexing a large number of web pages, that they then make searchable within their database.
A user comes to Google, Bing, or a number of other search engine websites, and inputs their search criteria.
The search engine then looks through its database, using an algorithm to determine which of the indexed websites the user most likely meant to find.
It then ranks these websites by a system of values based on the attributes of the websites.
For example, to be included in a search for term x, the website must include the term x in its content in some manner. However, that just gets it into the pool of results.
Where it falls in the ranking is determined by several other factors, one of which is how many other websites have links to the website in question.
The search engine then displays these results. Some websites attempt to boost their ranking within the search results by optimizing their content for the search engines algorithm.
This can be done by including the types of key words that the target audience of the website is likely to be utilizing in their searches. It can also be done by increasing the number of other websites that have links to the website in question. However, some search engines have rules about what kind of optimization is permissible.
One way that search engines determine the value of a website is by examining the number of other websites that have links to the website whose ranking the search engine is attempting to discern.
Natural links from one website to another are those that are created for the purpose of presenting the content in a useful way.
For example, a website about cameras might contain a link to another website where one can purchase similar cameras to those described on the first website.
This is a natural link. It exists for the purpose of making it easier for the user of the website to find cameras, which they were just reading about on the first website.
Unnatural links are those, which do not have this purpose.
Unnatural links often exist on websites with a large number of links to many websites, that the owners of the linked sites pay to have present on the linking website.
The website with all of the links exists only to have links, and there is no reason for this other than that it will increase the search engine rank of the website that is linked to.
Some search engines penalize the website for this by lowering its ranking in the search results.
The ranking in the search results is critically important because few people go very far into the pages presented by the engine, with many people stopping at the first page of results.