TeX (pronouced "tech", standing for Tau-Epsilon-Chi) is a typesetting software written by Donald E. Knuth, intended for the creation of beautiful books -- especially for books that contain a lot of mathematics (Knuth's own words).
LaTeX is a TeX macros package written by Leslie Lamport to relieve users of TeX of the burden of writing the document format before they can actually start writing the content of the document.
TeX is a very powerful typesetting package. But it also has a very steep
learning curve, and not many people can be motivated enough to learn it
to a level of competence to typeset a reasonably complex document. On the
other hand, LaTeX, which defines an extensive set of formatting macros, has
a gentler learning curve, and exploit the power of TeX to a level for a novice
to do fantastic things with LaTeX.