Many of the top search engines and even some directories rank web sites (and pages) according to how many links there are that point or connect to a site (mostly from other sites but some search engines also include higher quality links within your own site).Many of the top search engines and even some directories rank web sites (and pages) according to how many links there are that point or connect to a site (mostly from other sites but some search engines also include higher quality links within your own site).
These are called incoming links - (from an outside source which compares you to others sites and their links - "link popularity"). Google (and others) considers link popularity to be a very important factor in their ranking algorithm and Google has also provided its own term specifically for their gauge of your link popularity - PageRank (or PR) - a scoring system that determines how important a specific page is based on the developed links to that page.
A site with mostly incoming links has a high link popularity and as such is classed as an authority on a specific set of topics (generally takes a long time to develop this status). Search engines can also rank according to how many outgoing links a site has. A site that has many outgoing hyperlinks is considered a hub (or many links to highly relevant information). A hub may start with low link popularity but this can rapidly change. These outgoing links are usually to highly sought information in a similar area or field of interest and as such tend to develop an authority status by proxy since they aggregate a large amount of information so people need not look for it themselves - thus incoming links develop naturally and an authority hub is born.
Link Trading 
