Tracking if your email is opened is generally tracked by inserting a tiny, transparent graphic at the bottom of the message. When the tracker image is downloaded from the server after a recipient opens the email, it is counted as an open.
Because plain-text emails don’t contain images, open tracking is handled differently. Some services like MailChimp lets you enable click-tracking in your plain-text emails. When someone clicks a link in their plain-text email, we track that as an open and a click.
MailChimp automatically converts all links in your emails to point to our redirect scripts. When a recipient clicks your link, they’re redirected from our server to your original URL, and track the click. That helps us generate campaign reports that tell you which links your recipients clicked most.
To track clicks without using mailchimp you would embed analytics code into graphics, when the graphics is pulled from the server it would count it as an open.