In order to use LATEX you generate a file containing both the text that you wish to print and instructions to tell LATEX how you want it to appear. You will normally create this file using your system's text editor. You can give the file any name you like, but it should end ``.TEX'' to identify the file's contents. You then get LATEX to process the file, and it creates a new file of typesetting commands; this has the same name as your file but the ``.TEX'' ending is replaced by ``.DVI''. This stands for `Device Independent' and, as the name implies, this file can be used to create output on a range of printing devices. Your local guide will go into more detail.
Rather than encourage you to dictate exactly how your document should be laid out, LATEX instructions allow you describe its logical structure. For example, you can think of a quotation embedded within your text as an element of this logical structure: you would normally expect a quotation to be displayed in a recognisable style to set it off from the rest of the text. A human typesetter would recognise the quotation and handle it accordingly, but since LATEX is only a computer program it requires your help. There are therefore LATEX commands that allow you to identify quotations and as a result allow LATEX to typeset them correctly.
Fundamental to LATEX is the idea of a document style that determines exactly how a document will be formatted. LATEX provides standard document styles that describe how standard logical structures (such as quotations) should be formatted. You may have to supplement these styles by specifying the formatting of logical structures peculiar to your document, such as mathematical formulae. You can also modify the standard document styles or even create an entirely new one, though you should know the basic principles of typographical design before creating a radically new style.
There are a number of good reasons for concentrating on the logical structure rather than on the appearance of a document. It prevents you from making elementary typographical errors in the mistaken idea that they improve the aesthetics of a document--you should remember that the primary function of document design is to make documents easier to read, not prettier. It is more flexible, since you only need to alter the definition of the quotation style to change the appearance of all the quotations in a document. Most important of all, logical design encourages better writing. A visual system makes it easier to create visual effects rather than a coherent structure; logical design encourages you to concentrate on your writing and makes it harder to use formatting as a substitute for good writing.