LaTeX is a TeX macro package.  

The previous version of LaTeX was known as LaTeX2.09.  Over the years
many extensions have been developed for LaTeX. LaTeX2e brings all
extensions back under a single format and thus prevents the
proliferation of mutually incompatible dialects of LaTeX 2.09.  With
LaTeX2e the `new font selection scheme' is standard and, for example,
amstex (formerly the AmSLaTeX format) or slides (formerly the SLiTeX
format) are simply extension packages, all working with the same base
format.  The introduction of a new release also made it possible to
add a small number of often-requested features and to make the task of
writing packages and classes simpler.