Open Rate refers to the number of times the message is actually opened. It's not an exact science as some email programs may not load the image that is used to track the open rate and some email programs may not show the full HTML.
I just know that whatever I see for an Open Rate is probably a little bit low and I adjust my thinking accordingly.
It's nice to get an idea of your Open Rate because this gives you an idea of how many people on your list were at least interested enough to view your message. After all, if they don't read the message, they can't see your offer, right?
Tracking the click-throughs, however, is pretty precise because I use a unique AdTracker URL for each message that goes out. This lets me see exactly how many people clicked the link in my message and when.
I blasted out an Email Marketing message to one of my lists back on Dec 15, 2008. I was pretty happy with the Open Rate. It wasn't amazing but several hundred people did view the message.
What is more interesting to me, though, is that people are still clicking the link in that message today! I don't know if these folks just don't check their emails that often or if my message got saved for later, or what ... but I love knowing that a message I sent out weeks ago is STILL bringing me new potential customers today!