What is Linked Data?
Linked Data is a way of authoring a hypertext document in a way that makes the concepts and relations described in the document machine readable.
How is it used on the web?
The easiest way to see Linked-Data on the web is to use a tool that helps you see the information embedded into the web pages you use. These tools show you how machines can read the information embedded in web-pages, which can be used for many purposes; including referencing the machine-readable data in those pages in applications you might want to develop that can utilise this data, from all the webpages you reference in your application to create what is called a ‘graph’ of information about any particular subject.
Most major websites use linkeddata; but they’re not all using the same languages. Linked-Data languages are called ontologies. An ontology is a specially defined language or machine-readable dictionary, that’s been designed for some specified purpose. Websites such as wikipedia is made available as linked-data via wikidata.
For a more comprehensive list of linked-data ontologies the website LOD (linked open data) Cloud provides a navigable tool to find various ontologies for almost any purpose. Other tools like LinDA can be used to search for particular terms and find different ontologies that define those terms in a machine-readable way. The most popular ontology for search-engine optimisation is called schema.org and is continually developed via a github issues list and related W3C Community CG.