What is structured data?
Structured data (also called Schema Markup and Schema Data) is the markup of content on your website that allows Google to display information from the website (for example, contact information, reviews, events, product information, address, images, etc.) directly in search results. Structured data can be used, for example, on product pages to display price, stock status and rating:
Another example is recipes, where Google can display both an image and other information:
In other words, structured data can give you more visibility in search results and more return on your positions, because the search result is fuller and much more appealing for users to click on than if it did not use structured data.
Implementation of structured data
Every website may make use of some form of structured data. However, this is most relevant for e-commerce websites, companies with physical stores and websites with large amounts of media content (articles, recipes, videos, courses and more).
As a rule, structured data is implemented using Scheme.org (hence the name Schema Markup) - a joint collaboration between a number of the world's largest tech companies, including Google, Microsoft and Yahoo. Specifically, it works by inserting a piece of code around selected content on the website, indicating what the content is. For example, it could be a code indicating that here is a phone number.
At developers.google.com you can see many more examples of good use of structured data and read how to implement it in practice. Google offers the option, via Google Search Console, to tag data on your website rather than using structured data. However, this is not an appropriate solution because it does not help other search engines.