Great question. Trust is sort of a catch-all term that we use. So PageRank is the most well known type of trust. It’s looking at links and how important those links are. So if you have a lot of very high quality links, then you tend to earn a lot of trust with Google.
There are other signals. There are over 200 different signals that we use in our ranking. But you can kind of break them down into this notion of sort of trust and how well you match a particular query. So how topical you are. What’s your information retrieval score? Just on the merits of what the user typed in.
PageRank is trying to figure out, we use several words: reputation, trust, authority. And it’s not that we have, or we have something specifically called authority rank, or something like that. We’re basically just trying to say, in the general scheme of things, how much reputation, or how much are we willing to believe that this is a high quality page, or a high quality site? Those sorts of things.
So in general, what you want to have is a very reputable site. But you also want to have a site that’s about, or a page that’s about, the topic that the user typed in. So in an ideal world, you’ve got both. It’s very high on the reputation scale, but it’s also exactly what the user was looking for in terms of typing in and finding a match for what they were typing. So we use a lot of different words like trust, reputation, and authority. And Page Rank is a specific example of those sorts of things. But we mean it in a more general sense. It’s not, typically, a specific algorithm. It’s just trying to figure out how much, if a regular user saw this, maybe without even seeing a query at all, how much would they consider it a high quality page? How much would they consider it really useful, as far as being able to answer whatever questions that particular site was about? Hope that helps.
by Matt Cutts - Google's Head of Search Quality Team