LaTeX (most commonly pronounced Lay-teck) is a document preparation package which sits on top of TeX (Teck). The latter was designed to produce high-quality typesetting, with a particular emphasis on mathematical text commonly used within science. TeX is extremely powerful (ie flexible), but as a result can be rather difficult to use by the non-expert. LaTeX is an attempt to keep the flexiblity & functionality, but wrap some of the more commonly used functions into more user-friendly commands.
Further information on LaTeX is available in the LaTeX User's Guide & Reference Manual (paperback; ISBN 0-202-15790-X) by Leslie Lamport (Addison-Wesley Publishing Company).