is an RSS optimizing service that can do wonders for your plain old podcast feed. One of the things that it can do for you is provide some stats on how many subscribers you have. Remember, I said subscribers which only refers to listeners that grab your show via your feed as is the case with podcatchers. Listeners that come to your website to download your shows won't be included in Feedburner metrics.
However, feed stats are still a very useful thing to have and the way Feedburner works is this:
You direct your 'original' feed (meaning the feed that your blogware or host offers my default) to Feedburner. Feedburner then adds whatever bells and whistles you want to it, and gives you a new feed address that you are supposed to give out to your listeners. With this model you don't want your listeners to be subscribed to your original feed and since listeners aren't likely to switch just because you ask them too, most people feel that failure to set up Feedburner right in the beginning means that a show should never use Feedburner.
I agree with that because you take a very real chance of losing listeners and many of them will never come back. However, if you happen to be using Wordpress, then you can use the to magically redirect your 'original' feed to Feedburner to take advantage of all of Feedburner's features, and your listeners don't even have to know.