Metadata is structured data which contains information from a resource e.g: book, web document. With metadata the information-resource is easier to find because they contain general information such as: title, author, publishing date, description etc. They are used i.e. in libraries, but also for statistics and software engineering.
Metadata are used for HTML-programming to note information for server, browser and automated programs (robots). They can also contain instructions for redirection to another site. The definition of metadata is in the head of a HTML-document. All metadata are written in meta tags.
Metadata can be noted in different ways: all data is optional, but they are recommended by search engines and also by Online-Marketing experts. On one hand, search engines receive hints for the webdocument-content, but on the other hand users see e.g. the meta description as rich snippets in the SERPs. In individual cases it should be considered, which metadata should be noted. Here you can see a metadata-extract:
<head> <meta name="description" content="Dieser Beschreibungstext soll den Inhalt erläutern."> <meta name="author" content="Max Pattern"> <meta name="keywords" content="HTML5, Metadaten, Metatag"> <meta name="date" content="2014-12-15T08:49:37+02:00"> </head>
When creating metadata the dublin core can be used as a standard.
Search engine - crawler uses metadata to read meta-information out of web documents. This data helps to index and to assign search engine sites correctly. In HTML, information can be recorded by meta tags. Some meta tags e.g. title-tag are very important for search engines and are considered as a ranking factor.