What is a subscriber?

I noticed the Feedburner buttons that tell you how many subscribers a feed has.

When generating stats for a feed that you are serving (not through a third party), how would you tell when you have a subscriber?

 

google analytics i guess

 

I dind't used FeedBurner, but for others I've seen suscribers on "user agent" field of log files. Examples:
"Bloglines/3.1 (http://www.bloglines.com; 6 subscribers)"
"Feedshow/1.0 (http://www.feedshow.com; 5 subscribers)"
"NewsGatorOnline/2.0 (http://www.newsgator.com; 2 subscribers)"
"kinjabot (http://www.kinja.com; 7 Readers)"
"NewsAlloy/1.1 (http://www.NewsAlloy.com; 1 subscribers)"