Markup

Continuous speech, or computer data or characters, is a linear phenomenon. Markup is any system of distinguishing one segment of text from other segments which imposes an order on the underlying byte stream.

There are several different types of markup, ranging from the most appearance-oriented (physical) to the most structure-oriented (logical): (Coombs, Renear, and DeRose 1987) :

Punctuational: Punctuational markup includes commas, dashes, and semicolons. We often think of these as an integral part of text, but some ancient languages were written without any punctuation whatsoever, including spaces.
Presentational: Presentational markup includes horizontal and vertical spacing such as page breaks, double line spacing, and indentation. Presentational markup is what is generated with a mechanical or electric typewriter.
Procedural: Procedural markup takes the form of instructions to a computer program that will then generate presentational and/or punctuational markup. Languages such as TEX and Postscript are often considered procedural markup languages.
Descriptive: Descriptive markup concerns the logical structure of a document rather than its appearance. Descriptive markup is designed to be easy for both people and computers to read and understand. User-defined styles in word processors can be used for descriptive markup.
Metamarkup: Languages that can generate various types of descriptive markup are considered "metamarkup" languages. Standard Generalized Markup Language (SGML) is probably the best example in this category.

These types of markup are not exclusive. Punctuational and presentational markup are somewhat independent, but one can often serve as a substitute for the other (such as when lists separated by commas are replaced by bulleted lists.) Procedural markup can produce all of the effects on the page that descriptive markup can, but it loses (theoretically irrecoverably) the information about the logical structure of the document provided by descriptive markup. With metamarkup, you can generate specific markup languages that will meet particular needs.

Even though punctuational and presentational markup could be replaced with appropriate descriptive markup generated by metamarkup, it is hard to imagine people actually encoding sentences as

<sentence>the name of my dog is
<propernoun>rover</propernoun>
</sentence>

even if computer processing would be easier as a result. If we ever do get to that point it is just one step away from writing completely in terms of the logical significance of the document; such documents might look like

<sentence><defnarticle>
<name1><of3><pronounfirstsingposs>
<canine2informal><tobe>
<propernoun>rover</propernoun>
</sentence>

where the only remaining text is text that refers to items not yet represented inside the computer, such as my dog Rover.

A more likely alternative is that the "period" key on the keyboard would actually generate a <sentence></sentence> pair, but even that remains unlikely for the near future. Punctuational markup at least will be with us for a while.

There is another category of markup, referential markup, that "refers to entities external to the document" (Coombs, Renear, and DeRose 1987) but it describes a different aspect of markup than the previously listed categories. Referential markup is the basis of hypertext.