I just pruchased a domain name with a rank off around 100,000. In alexa's rankings is it how much traffic the domain name gets? For instance, I am going to develop a brand new site, will I keep that same alexa ranking?
Alexa rates by traffic. If traffic falls so does rank. Some sites use scripts to get higher rank so alexa is not the best way to see how good a site is. Make sure to keep track of visitors and unique visits. If you keep site same layout with same topics it could stay same rank
Alexa is irrelevant, PR is maintained and managed better by the owners (google) and is more trust worthy of your site's traffic then say Alexa which isn't maintained nearly as good as Google's PR.
As I understnad it alexa is a ranking, so the lower yor number the more traffic the site gets. I think it's calculated based on how many visitors the site gets, particularly those visitors that have the alexa toolbar installed. As a consequence if your website is directed at a niche that is frequently by net-savvy people with the toolbar installed it will get a better alexa ranking than a site of a niche that isn't used by net-savvy people.
I wouldn't solely trust alexa for an accurate traffic rating. Pr is also suspect now with google downplaying its importance. I think google should get a system going where it takes the top 5-10 keywords that are searched for each particular website and give rankings to websites based on the total traffic to the site from those keywords. The rank would not only reflect numbers to the site, but quality of traffic given the content of the site.