It's a small transparent pixel loaded from a website (assuming the email client loads HTML). I'd make a unique image called mywebsite.com/jaclaz_read.png and when that image was loaded I'd know you read my email.
I am particularly "old style", I know, but I tend to use not whenever possible email clients that load the html, so an old habit turns out to be also (at least partially) "safer".
Of course in the US a lot of practices revolving around "controlling employees" are allowed and even considered "normal" whilst in Europe they are seriously frown upon.