LaTeX is a set of macros that runs on top of TeX providing a very easy to use logical markup language. LaTeX is not a desktop publishing package. It is meant for longer documents that follow a regularized style. The author of the document focuses on the content of his document while the typesetter writes a style that, combined with the LaTeX document, will produce a TeX document that is then transformed into a DVI or PostScript document for printing. LaTeX has all the strengths of TeX with the additional strengths of using logical, instead of physical, markup. LaTeX can even be used to write simple programs (although it isn't very pretty).
The only real disadvantage of LaTeX is that document writing is done with markup to plain text, so one cannot see what his final output will look like until he processes the document with the LaTeX parser. However, there are several good GraphicalUserInterface programs like LyX that allow one to edit a LaTeX document graphically.
See the LaTeX Project Home Page for more detailed information.