LaTeX is a TeX macro package, originally written by Leslie Lamport, that provides a document processing system. LaTeX allows markup to describe the structure of a document, so that the user need not think about presentation. By using document classes and add-on packages, the same document can be produced in a variety of different layouts.
Lamport says that LaTeX "represents a balance between functionality and ease of use". This shows itself as a continual conflict that leads to the need for such things as FAQs: LaTeX can meet most user requirements, but finding out how is often tricky.