When emails are delivered from our system, we automatically add a tiny piece of code which requests a tiny invisible image from our servers. When a reader opens the email and downloads the image we record that download and add that as an open for this specific email.
We also record opens based on people who click on links within your campaign. Even if they don't download the images we record this as an open because we know they read some of your email.
For more information on open rates & tracking check out this Wikipedia Article https://en.wikipedia.org/wiki/Open_rate
You may read that open rates are not a true reflection of who has opened your campaign and although this is true (please refer to this article). We believe that using today's technology we can say that it's more accurate than many people think.
At the end of the day email opens and open rate should not be the only figure driving your strategy or campaigns. There are many other statistics that should be looked at to get a clear understanding of how successful your campaigns are.