Microdata

HTML Microdata defines how machine-readable data can be embedded in HTML documents so that it is recognized by the major search engines. It's similarity to RDFa and Microformats can be seen in the Rich snippets (microdata, microformats, RDFa) examples provided by Google ( e.g. Recipes) . The article Microdata, semantic markup for both RDFers and non-RDFers outlines how Microdata relates to RDF.

Google's Schemas in Gmail allows JSON-LD and Microdata, to be used to Embed Schemas in Emails. Email information can be marked up, enabling Google to understand the fields and provide the user with relevant search results, actions, and cards. For example, if the email is about an event reservation, you might want to annotate the start time, venue, number of tickets, and all other information that defines the reservation.

The HTML Data Guide provides guidance for both the publishers and consumers of HTML data.

Microdata to RDF describes how to transform HTML Microdata to RDF.

Schema.org

Schema.org provides a collection of microdata schemas, i.e. html tags, that webmasters can add to their pages to describe the things their pages are about. Getting Started provides a tutorial.

The articles Structured Web Gets Massive Boost and Schema.org - Threat or Opportunity? both provide good overviews on how Microdata and Schema.org relate to RDF, Linked Data and the Semantic Web.

Tools

References