DTDs vs Well-Formed Documents. An Example of a Well-Formed Document. A More Powerful DTD-based Document. Marking up is the process of adding tags to textual data. Tags are easy to read, write and understand by you and any computer.
Tags can add formating information to a document and structural information to data.
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.