Tags are presented inline with information. Tags have a consistent format and are designed to be human-readable. SGML stands for Standard Generalized Markup Language. Documents that describe tags sets are called Document Type Definitions, or DTDs. In DTDs, you can specify what tags contain what other tags, number of tags allowed, etc.
Documents are objects.
These people can't afford to convert all their files every time a new version of Microsoft Office comes out. HTML stands for Hypertext Markup Language. HTML was designed to describe documents. XML stands for Extensible Markup Language. XML was designed to describe structured data.