Hi All!
Continuing my series of discussions about the various keyword scores we use here at Moz (previously: Keyword Difficulty & Keyword Opportunity)... Let's move on to Volume.
Volume in Moz's tools is expressed in a range, e.g. Bartending Certification has volume of 201-500. These ranges correspond to data we have suggesting that in an average month, that keyword is searched for a minimum of X to a maximum of Y (where X-Y is the volume range). We use clickstream data as well as data from Google AdWords and then some PPC AdWords campaigns we run and have access to when we build the models for our volume data. As such, we've got very high confidence in these numbers -- 95%+ of the time, a given keyword's monthly search volume on Google will fall inside that range.
If you want to see all the nitty gritty details, check out Russ Jones post on Moz's Keyword Volume and how we calculate it.
As far as a "good" volume score -- higher is usually better, as it means more demand, but lots of keywords with low volume scores can also add up to strong traffic when combined, and they may be more relevant. Capturing exactly the audience you want that also wants you is what SEO is all about.
p.s. When Keyword Explorer or Moz Pro gives you a "no data" or "unknown" volume number, it may just mean we haven't collected information from our clickstream providers or AdWords crawls, not that the keyword has no volume (though it sometimes means that, too, we just don't know yet). One way to verify - see if Google Suggest autofills it in when you type in the search box. If it does, that's usually a sign there's at least some volume (even if it's only a few searches a month).