Text processing programs use explicit commands to accomplish the same thing. In general, it is easier to make major changes to the format of a document created with a text processor than it is with a word processor. Text processors are generally more powerful. Detractors of these programs say what you see is all you get. There are preview options for LaTeX and certain versions of troff, but these are run after the text is processed.
Word processors are better suited for short documents such as letters or memos.
However, LaTeX can be used for letters and MS Word has been used to write a book.