LaTeX is a document preparation system that enables the document
writer to concentrate on the contents of their text, without bothering
too much about the formatting of it. For example, chapters are
indicated by \chapter{<title>} rather than by selecting 18pt bold.
The file that contains the information about how to turn logical
structure (like `\chapter') into formatting (like `18pt bold ragged
right') is a document class. In addition, some features (such
as colour or included graphics) are independent of the document class
and these are contained in packages.
One of the largest differences between LaTeX 2.09 and LaTeX2e is in
the commands used to write packages and classes. In LaTeX 2.09,
there was very little support for writing .sty files, and so writers
had to resort to using low-level commands.
LaTeX2e provides high-level commands for structuring packages.
It is also much easier to build classes and packages on top of each
other, for example writing a local technical report class cetechr
(for the Chemical Engineering department) based on article.