A backlink is an incoming link to a website/webpage. The number of backlinks is an indication of the popularity/importance of that website or page. They may be of significant personal, cultural or semantic interest: they indicate who is paying attention to that page. Backlinks enable you to keep track of other pages on the web that link to your posts.
In basic link terminology, a backlink is any link received by a web node (web page, directory, website, or top level domain) from another web node. Backlinks are also known as incoming links, inbound links, inlinks, and inward links. There are several factors that determine the value of a backlink. Backlinks from authoritative sites on a given topic are highly valuable. If both sites have content geared toward the keyword topic, the backlink is considered relevant and believed to be have strong influence on the search engine rankings of the webpage granted the backlink. A backlink represents a favorable 'editorial vote' for the receiving webpage from another granting webpage. Another important factor is the anchor text of the backlink. Anchor text is the descriptive labeling of the hyperlink as it appears on a webpage. Search engine bots (i.e., spiders, crawlers, etc.) examine the anchor text to evaluate how relevent it is to the content on a webpage. Anchor text and webpage content congruency are highly weighted in search engine results page (SERP) rankings of your webpage with respect to any given keyword query by a search engine user.
Search Engine Optimization
Websites often employ various techniques, called Search Engine Optimization(SEO) to increase the number of backlinks pointing to their website. In other words, SEO is the process of improving the volume and quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results. Usually, the earlier a site is presented in the search results, or the higher it "ranks," the more searchers will visit that site. SEO can also target different kinds of search, including image search, local search, and industry-specific vertical search engines. Quality backlinks are crucial for SEO's success. But the the question is how to get them..?
The idea behind including backlinks as part of the page rank algorithm is that if a page is good, people will start linking to it. And the more backlinks a page has, the better. But in practice it is not exactly like this. Even if plenty of backlinks come to your site the natural way, additional quality backlinks are always welcome and the time you spend building them is not wasted. Among the acceptable ways of building quality backlinks are getting listed in directories, posting in forums, blogs and article directories. The unacceptable ways include inter-linking (linking from one site to another site, which is owned by the same owner or exists mainly for the purpose to be a link farm), linking to spam sites or sites that host any kind of illegal content, purchasing links in bulk, linking to link farms, etc.
There are different ways to for SEO. Go for volume of links or go for Trusted links. But only a few go on the angle of "get the right links" and you might not need as many links as you thought you did. It should be noted that a blog with the right 10 links can beat the blog with 1000 of the wrong links.
A typical search engine results page
The above pictures shows the difference between the site that has lots of wrong links from many wrong sites and sites that has few right links from trusted sites like google, yahoo etc.
PageRank
PageRank is a numerical value which represents the popularity of a website. It is one of the methods Google uses to determine a page's relevance or importance. It is from 0 - 10, with 10 being the highest number awarded.
"Mathematical PageRanks (out of 100) for a simple network (PageRanks reported by Google are rescaled logarithmically). Page C has a higher PageRank than Page E, even though it has fewer links to it: the link it has is much higher valued. A web surfer who chooses a random link on every page (but with 15% likelihood jumps to a random page on the whole web) is going to be on Page E for 8.1% of the time. (The 15% likelihood of jumping to an arbitrary page corresponds to a damping factor of 85%.) Without damping, all web surfers would eventually end up on Pages A, B, or C, and all other pages would have PageRank zero. Page A is assumed to link to all pages in the web, because it has no outgoing links."
According to Google "PageRank relies on the uniquely democratic nature of the web by using its vast link structure as an indicator of an individual page's value. In essence, Google interprets a link from page A to page B as a vote, by page A, for page B. But, Google looks at more than the sheer volume of votes, or links a page receives; it also analyzes the page that casts the vote. Votes cast by pages that are themselves "important" weigh more heavily and help to make other pages "important"."
According to Wikipedia.com "PageRank is a family of algorithms for assigning numerical weightings to hyperlinked documents (or web pages) indexed by a search engine. Its properties are much discussed by search engine optimization (SEO) experts. The popular search engine Google to help determine a pageĆ¢€™s relevance or importance uses the PageRank system. GoogleĆ¢€™s founders Larry Page and Sergey Brin developed it while at Stanford University in 1998." However, examine search results and you will find that page rank isn't the only determining factor in search engine placement, although it has value and is part of the algorithm.
According to webworkshop.net, "PageRank is a numeric value that represents how important a page is on the web. Google figures that when one page links to another page, it is effectively casting a vote for the other page. The more votes that are cast for a page, the more important the page must be. Also, the importance of the page that is casting the vote determines how important the vote itself is. Google calculates a page's importance from the votes cast for it. How important each vote is is taken into account when a page's PageRank is calculated. PageRank is Google's way of deciding a page's importance. It matters because it is one of the factors that determine a page's ranking in the search results. It isn't the only factor that Google uses to rank pages, but it is an important one.
Note that, not all links are counted by Google. For instance, they filter out links from known link farms. Some links can cause a site to be penalized by Google. They rightly figure that webmasters cannot control which sites link to their sites, but they can control which sites they link out to. For this reason, links into a site cannot harm the site, but links from a site can be harmful if they link to penalized sites. So be careful which sites you link to. If a site has PR0, it is usually a penalty, and it would be unwise to link to it."
Page Rank : Some Facts
1. One-way inbound links from websites with topics that are related to your website's topic will help you gain a higher page rank.
2. Other one-way inbound links from pages with high page rank but unrelated topics do help a little, but not nearly as much.
3. The number of links outbound from the website that links to you also determines the value of the link. A related website with 10 outbound links that links to you is much better than an unrelated website with 100 outbound links that link to you.
You can find more information on this topic from :
* What are Backlinks and how do i use them?
* Wiki : Backlink
* Improving your page rank