Compared to word processors
Most people have used a word processor, so a comparison may be helpful.
With a word processors your text is placed while you type it, referred to as "what you see is what you get." In contrast, TeX is a formatter: it separates the steps of entering the material and placing it on the page.
To see the difference, consider how a typical user of each system might start a new section. In a word processor a typical user might start that section by hitting
Beginners like word processing but when they graduate to complex jobs the appeal fades. Word processing a twenty page technical article is hard; for instance, keeping the vertical space between sections uniform is error-prone, and so is making sure that all of the bibliographic entries follow the required format. In particular, very few people have both the knowledge and the eye to correctly lay out equations — people often say their equations "just don't look right." That is, as a user becomes more experienced and knowledgable the TeX approach of having the typesetting done by the program becomes the better choice. (Some word processors offer as advanced features TeX-like facilities for organizing input text, although few users take advantage of them.)