Schema.org is a collaborative, community activity with a mission to create, maintain, and promote schemas for structured data on the Internet, on web pages, in email messages, and beyond.
Schema.org – http://schema.org
Microdata is a specification to embed machine-readable data in HTML documents. Microdata consists of name-value pairs (known as items) defined according to a vocabulary. A collection of commonly used markup vocabularies are provided by schema.org
Source – https://developers.google.com/schemas/formats/microdata
JSON-LD is an easy-to-use JSON-based linked data format that defines the concept of context to specify the vocabulary for types and properties.
Source – https://developers.google.com/schemas/formats/json-ld
Testing your schema markup
The Structured Data Testing Tool (SDTT) provides a variety of ways to develop, test, and modify your structured markup. You can load markup into the tool either directly or by supplying a URL.
The validator lets you check all the most common microformats: microdata, schema.org, microformats, OpenGraph and RDF.