Marketers and advertisers always want to know more about you, especially how you interact with the stuff they send you. Email tracking isn't new, but if you don't or haven't taken steps to prevent it, chances are it'll still happen to you.
It's pretty easy for any email sender to know when you've opened an email they sent you and also glean other information, such as where you are, whether you've clicked a link, and the kind of device you were using. It involves inserting a transparent 1 pixel by 1 pixel image into the email, and when the email is opened, the website the image is hosted on is notified of these details. I used to work in marketing, and that was how we tracked newsletter opens, and today there are many CRM products that do the same, including Streak and Yeswear.
There's nothing inherently bad about marketers gathering this data--except that many people might not be aware it's happening and didn't consent to it. If you don't care for others to know when or where you've opened their emails, Wired notes a new extension called Ugly Mail that warns you if tracking pixels are present in an email from one of the big three tracking services: Streak, Yesware, and Bananatag. PixelBlock, however, does more than just warn you; it blocks the tracking attempt and works with more sources.
If you don't want to install yet another Chrome extension, however, the best solution may be to turn off automatic image loading in Gmail. This will make emails load more quickly while also protecting your privacy. Who needs images anyway? At least transparent snooping images.