And what's more, since everyone followed the same standard of marking up, you could easily pick up a used text book and get a good idea just from reading the highlighted sections what were core points of the book. There are two crucial points to take away from this example. In the example above, markup is defined as a bit of yellow ink atop text. In HTML a markup is a tag. In HTML each tag communicates its own layout of formatting meaning.
Markups are also ubiquitous in the world of computers.
They are used by word processors to specify formatting and layout, by communications programs to express the meaning of data sent over the wires, by database applications that must associate meaning and relationships with the data they serve, and by multimedia processing programs which must express meta-data about images or sound. HTML is one of the more famous computer markup systems. HTML defines a set of tags that associate formatting rules with bits of text. Actually, though HTML is often called a programming language it is really not. Programming languages are 'Turing-complete', or 'computable'.