Jon Galloway explains how Google changed their ranking system:
Jagger's [Google index update from October 2005] main change is the switch from the elegant but overly trusting PageRank system to the more realistically cynical TrustRank system, which is designed to only count votes from sites it trusts.
TrustRank imitates human behavior - if a stranger on a train recommends a movie, I'm going to value it a lot less than a recommendation from a close friend or movie critic, both of whom have earned my trust by either how long I've known them or by their reputation. Trust comes from two sources - site age and links from trusted sources. From my movie recommendation analogy above, site age is the close friend who has gained trust through the age of the relationship, whereas trusted sources are sites who has been granted a position of authority by links form a small seed group of trused sites.
Another way to look at this is from the point of view of a content publisher with a new site. At first, your links will be untrusted and will not contribute to the Page Rank of the page they link to. The site has to undergo an aging delay to before it is considered authoritative, which has led to discussion of the "Sandbox" (or the "Trustbox"). The idea is that new sites are sandboxed so they can't mess up the rankings until they've proven themselves, at which time they can participate in Page Rank voting.
There are two ways to gain trust and escape the Trustbox:
* Acquire links from highly trusted sources (the "movie critic recommendation")
* Acquire links from somewhat trusted sources and let them age (the "friend recommendation")
Google Sandbox is a filter whose criteria is the age of a site. After let's say 4-6 months or when the site acquires highly trusted links, a site is given credit for what it has achieved, for the backlinks it has established: its PageRank increases and it's more visible in the search results.
Related:
Expertrank: authoritative search
The future of search