A WordPress theme consists of template files with code that formats the content. Most themes have a template file called footer.php, which defines how the footer is created.

For example, it’s impossible to install WordPress plug-ins on a free WordPress.com blog. And although you can customize a free website’s layout, you can’t install your own custom template ...

I believe most of us find going to Appearance > Themes > Add New and selecting your zipped theme file the easiest way to install a WordPress theme.