The first major difference between LaTeX 2.09 style files and LaTeX2e packages and classes is that LaTeX2e supports modularity, in the sense of building files from small building-blocks rather than using large single files.
A LaTeX package or class can load a package as follows:
\RequirePackage[<options>]{<package>}[<date>]
For example:
\RequirePackage{ifthen}[1994/06/01]
This command has the same syntax as the author command \usepackage.
It allows packages or classes to use features provided by other
packages. For example, by loading the ifthen package, a package
writer can use the `if...then...else...' commands provided
by that package.
A LaTeX class can load one other class as follows:
\LoadClass[<options>]{<class-name>}[<date>]
For example:
\LoadClass[twocolumn]{article}
This command has the same syntax as the author command \documentclass.
It allows classes to be based on the syntax and appearance of another
class. For example, by loading the article class, a class writer
only has to change the bits of article they don't like, rather than
writing a new class from scratch.
The following commands can be used in the common case that you want to simply load a class or package file with exactly those options that are being used by the current class.
\LoadClassWithOptions{<class-name>}[<date>]
\RequirePackageWithOptions{<package>}[<date>]
For example:
\LoadClassWithOptions{article}
\RequirePackageWithOptions{graphics}[1995/12/01]