The first thing you will notice in the toolbar is a green bar that indicates PageRank. To find the specific numerical value associated with the green bar you can hover the mouse over it. PageRank is Google's version of link popularity and takes the weight and context of incoming links into account. The amount of PageRank needed for each individual level increases exponentially. It is much easier to go from a rank of 3 to 4 than to go from 5 to 6. In fact one or two good links could very well give a page a rank of 4, however you might need a thousand to get a 6. Its very hard to figure out exactly what the formula is, but research done by myself points to the base being a 4 or a 5. So each level would be 4 or 5 times as hard to reach as the one previous to it.
It is important to note that the value given in the toolbar is not always accurate. If you are visiting a page that Google has not yet spidered, but they have spidered the root domain of the site, then they will guess a PageRank based on the distance from the root of the site to your current position. This is only a guess and has no bearing on ranking. Once Google spiders the page they will assign an actual PageRank to it. This guessing behavior is the reason why some Geocities pages seem to have high PageRanks, the fact is they've simply not been spidered yet.
The other thing the toolbar is specifically useful for is that it allows you to easily check backwards links. Directly to the right of the PageRank bar you should notice a "Page Info" button, if you click on this you should see an option for backward links. Selecting that will give you a list of all the links Google has in it's public index. You will run into cases where a page has a rank in the PageRank bar but you can find no backward links. This can happen for two reasons. The first is that Google has not spidered the page. Sometimes when Google has not spidered a page the toolbar will guess a ranking based on the domain (assuming it has spidered the domain) this ranking is really just a guess and has no weight at all. This is why pages on free hosts such as Geocities seem to have a high a PageRank, when in reality they've just never been spidered.
The other reason a ranked page may show no backwards links is that Google does not put every page it spiders into it's public index. If a page itself does not have a very high PageRank, perhaps because it only has one inbound link or something of that nature, then while Google will note and count the outward links on that page, the page itself will not be listed in the index. This is evident in the various popular directories like DMOZ or Yahoo. If you have a directory listing in an obscure subcategory then that listing will indeed raise your rank but the subcategory itself may not be listed in the index.
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