I saw the following "Track order" link in Flipkart's order confirmation mail (gmail):
How do I acheive this? I don't know what it is called, so don't know what to search for. I apologize if this question is already asked.
This is done using an Email Schema via schema.org.
Google uses this service to allow rich markup of emails (https://developers.google.com/gmail/markup/overview)
You will need to register with Google and comply with their guidelines in order to use a schema in an email to provide a deep-link to your website.
The guidelines are located here: Registering with Google
In Summary:
There's a few more, but these are the main requirements. Once you comply, you need to email [email protected] to allow Google to ensure you comply with the requirements and white list your domain for deep-linking.