It offers programmable desktop publishing features and extensive facilities for automating most aspects of typesetting and DTP, especially numbering and cross-referencing, tables and figures, page layout, bibliographies, and much more. LaTeX was originally written in 1984 by Leslie Lamport and has become the dominant method for using TeX; few people write in plain TeX any more. The current version is LaTeX2ε.
(Historical note: LaTeX is pronounced LAY-tekh (SAMPA: ["leitEx]) or LAH-tekh ("latEx) where kh represents the sound of ch like in German ach or Scottish loch, and traditionally printed . TeX itself was written by Donald Knuth in 1978 and provided the world's first program to produce publishable quality typesetting that could be run from the desktop terminal of a normal business computer.)
LaTeX is based on the idea that authors should be able to concentrate on writing within the logical structure of their document, rather than spending their time on the details of formatting. It encourages the separation of formatting from content, whilst still allowing manual typesetting adjustments where needed. By keeping the formatting details in a separate file from the text, it is often regarded as much superior to word processors and most other desktop publishing systems.
These allow trivially easy visual layout changes, but intertwine content and form so tightly that consistency is often difficult, and automation of longer or more complex documents virtually impossible. LaTeX also provides great flexibility in formatting while maintaining the identity of structure, which purely structural systems like SGML and XML do not directly address. LaTeX can be arbitrarily extended by using the underlying macro language for developing custom formats.
LaTeX was originally most commonly used by mathematicians and scientists, amongst whom it remains the favored tool for writing papers, preprints, and books. Because of the underlying TeX system, originally developed for documents with mathematics, laying out mathematical expressions is considered to be easier, and the resulting typesetting of higher quality, than any competing document-processing systems. Many scientific journals and other publishers provide free LaTeX packages which implement their "in-house" typesetting styles. The Wiki markup for formulae uses the same system as LaTeX.
The popularity of LaTeX in the technical and academic communities is perhaps partly due to its early availability on Unix systems, and the comparative unavailability of competing word processors on those platforms until recently. But from an early stage LaTeX was available on a wider range of hardware and software than any other program, and versions are now available for almost any system from PDAss to desktop PCs to supercomputers. LaTeX is less popular than mainstream desktop publishing software outside the technical communities for several reasons. It is regarded as hard to learn for people with no previous experience of markup languages. Although it is very easy to customise the appearance of articles, books, and reports, using only a handful of instructions, it remains basically a typesetter for automating document production, not a manual page design program, so performing complex visual layouts incorporating multiple images is difficult. Another barrier to usage for many is the asynchronous interface used in most free versions, where editing is done in a different window to the typeset display. Several commercial implementations, however, use a synchronous typographic display like other DTP systems (as does LyX).
LaTeX is free software. It has a peculiar license called LPPL, not compatible with the GNU General Public License, that allows redistribution and modification, but requires that modified files carry a modified filename. This ensures that files that depend on other files will produce the expected behavior and avoids problems similar to DLL hell. A new version of the LPPL that will be compatible with the GPL is in the works. There are numerous commercial implementations of the whole TeX system (which includes LaTeX), and vendors may offer extra features like phone support and additional typefaces.
LyX is a visual document processor that uses LaTeX for a back-end. A number of popular commercial DTP systems use modified versions of the original TeX typesetting engine. The recent rise in popularity of XML systems and the demand for large-scale batch production of publication-quality typesetting from such sources has seen a steady increase in the use of LaTeX.