the basic premise, is that the url the image points to is not just an image, but an actual backend script that logs users, and if when you generate the email you assign each email an unique id to append to the trackback url, you can track who opened it.
sort of
<img src='http://www.mysite.com/track.php?src=w3223nsfiw92'>
from that you can track via key pair values which email opened it,
time & date
ip address of request.
once you have that you can track who opened it, when, and how many times.
harest part is generating emails with unique key pairs, since sending out thousands of emails via your own app is often blocked by isp's since it's technically spamming, so going with a service like constant contact is usually the best bet.