The most important difference between LaTeX 2.09 and LaTeX2e is the way that fonts are selected. In LaTeX 2.09, the Computer Modern fonts were built into the LaTeX format, and so customizing LaTeX to use other fonts was a major effort.
In LaTeX2e, very few fonts are built into the format, and there are
commands to load new text and math fonts. Packages such as times or
latexsym allow authors to access these fonts.
This document describes how to write similar font-loading packages.
The LaTeX2e font selection system was first released as the `New Font Selection Scheme' (NFSS) in 1989, and then in release 2 in 1993. LaTeX2e includes NFSS release 2 as standard.