By itself it will loop over all the blog categories in alphabetic order. If nested inside a <Post> tag, it will only loop over the categories to which that post belongs to.
from: From what category to start (default: 1)
count: How many to show (default: if omitted, if will loop over all categories, starting from "from" attribute)
ifCountGT: Only loop if there are more than this number of categories
ifCountLT: Only loop if there are less than this number of categories
loopAtLeastOnce: Render what's inside this tag at least once, even if there are no categories