A widely used document markup language which allows precise control over how a document is laid out. Written as LaTeX, this language is widely used in technical and academic papers for it's various features, including hyperlinks, references, table of contents generation, bibliographies, indexing, and the ability to compile into PDF files.
LaTeX is a macro package implemented on Don Knuth's TeX programming language and is used primarily for creating high quality written documents. The underlying TeX system is Turing complete and LaTeX inherits this property, making it possible (but rarely advisable) to perform arbitrary computations in LaTeX; more interestingly it makes it possible to get any layout scheme you can describe precisely.
There is a Stack Exchange site dedicated to LaTeX (and TeX, and other related technologies), which is a good place to ask any LaTeX-related questions.
A great many problems can be solved by selecting the right existing macro package from CTAN.