In this article Campaign Monitors' Matthew Patterson explains what open rates are all about.
Basically an open rate is a measure of how many people on an email list open (or view) a particular email campaign. The open rate is normally expressed as a percentage, and is usually calculated as follows:
So a 20% open rate would mean that of every 10 emails delivered to the inbox, 2 were actually opened.
How do you measure an open?
When each email is sent out, automatically a piece of code is added that requests a tiny, invisible image from the sender's web servers. So when a reader opens the email, the image is downloaded, and that download is recorded as an open for that specific email.
It is important to understand that the open rate is not a 100% accurate measure. Recording an 'open' can only happen if the readers email client is capable of displaying html with images, and that option is turned on. So if you are sending text-only emails, there is no way to record open rates (the exception is if they actually click a link). Similarly, people reading your html email without images showing will not be recorded as opens.
Another issue is that your readers may have a preview pane in their email client. That preview pane might be displaying your email automatically (and therefore downloading the images) without the reader ever having to click on it or read it.
So you should never take your open rate as a hard and fast number, because you can never know the true figure. It is much better used as general guide, and as a way of measuring the trends on your email campaigns.