To publish something, authors give their typed manuscript to a publishing company. One of their book designers then decides the layout of the document (column width, fonts, space before and after headings, ...). The book designer writes his instructions into the manuscript and then gives it to a typesetter, who typesets the book according to these instructions.
A human book designer tries to find out what the author had in mind while writing the manuscript. He decides on chapter headings, citations, examples, formulae, etc. based on his professional knowledge and from the contents of the manuscript.
In a LATEX environment, LATEX takes the role of the book designer and uses TEX as its typesetter. But LATEX is ``only'' a program and therefore needs more guidance. The author has to provide additional information to describe the logical structure of his work. This information is written into the text as ``LATEX commands.''
This is quite different from the WYSIWYG3.2 approach that most modern word processors, such as MS Word or Corel WordPerfect, take. With these applications, authors specify the document layout interactively while typing text into the computer. They can see on the screen how the final work will look when it is printed.
When using LATEX it is not normally possible to see the final output while typing the text, but the final output can be previewed on the screen after processing the file with LATEX. Then corrections can be made before actually sending the document to the printer.