The short answer is "we don't know". They're calculated and compiled by iTunes, we can only assume they're based on downloads. Given that the published charts are presented as "live" rather than covering any specified timeslot, there's obviously some kind of algorithm at play, but beyond that it's anybody's guess. Podcast charts are slightly different since these are charts of "channels" producing content on a rolling basis, rather than a finished product like an album or a song. Again, the chart is most likely based on downloads but may also be based on new subscriptions too.