Markup involves adding of characters to a piece of information such that these characters can be used to process that information in a particular way. A markup language is not a formatting language. Formatting language focuses on describing the final display of a document without trying to describe the documents content.
The best example of formatting languages is RTF that is rich text format. RTF was created specially to describe the final format of a document using codes embedded in a document. These codes are readable through a wide variety of applications from word processors to page layout programs, but are generally invisible to the document developer. It is extensively used in professional web design works.
A markup language is not a programming language since a programming language creates a set of instructions that are interpreted for compiled into a program or application. The best example of programming language is Java. Java applets are miniature applications that can run only inside other applications such as web browsers that undertake Java. In brief the programming language generates programs or applications, but markup languages generate documents. In fact markup is interpreted by a program.
Two different types of markup languages are used in professional web design namely procedural markup and descriptive markup. The best example of procedural markup is a rich text format file. It is procedural not because of its arcane syntax but because of the insertion of the layout information between the contents, parts of the document. This means that we have to search and replace these codes in order to port the document to another application. Also in between, media types for example, publishing the document on websites.
Further more a lot of subtle distinctions between different logical parts of the documents are lost because they carry the same procedural markup, they have same visualization. For example let us say some one assigns the same layout characteristics to some important words and words that are defined in glossary would appear slanting in the document.
There is no easy way to distinguish emphasized words and glossary items any more because they carry same procedural markup. In professional web design, procedural markup language is chosen with great deal of care. Descriptive markup is bit like Hyper Text Markup Language. The syntax of Hyper Text Markup Language like markup is quite readable. The use of angle brackets for designating the start and end of element is apparently easy to understand. Elements can nest also which can be derived from the order in which the different tags appear.
Hyper Text Markup Language is a mixture between procedural and descriptive markup. So it is extensively used in professional web design.