What

The World Wide Web is also known as the web, WWW or W3.

In an http url, the first substring that follows the initial http:// or https:// is called the domain name. Proceeding this is either www or nothing. Not using www the domain is referred to as naked.

The World Wide Web is also known as the web, WWW or W3.

Why 

Hypertext Transfer Protocol http was invented alongside html to create the first interactive, text-based web browser: the original World Wide Web. Today, the protocol remains one of the primary means of using the Internet.

www technically is a subdomain traditionally used to indicate that a site is part of the web, as opposed to some other part of the Internet like Gopher or FTP.

So a website might be www.forestpathways.co.uk the www indicating it is part of the web.

and ftp.forestpathways.co.uk the ftp (file transfer protocol) indicting it is a standard network protocol used to transfer of files from one host to another over a TCP-based network, such as the Internet. FTP works by opening two connections that link the computers trying to communicate with each other.

Whilst the original websites would have been all www prefixes today for branding reasons the internet has matured which is one reason the www has been dropped. But the choice is yours and it makes no difference for SEO.

How

Forestpathways.co.uk does not use the www prefix.

To do this and to ensure that even if someone enters www.forestpathways.co.uk the www is redirected forcing the non www.

Here is the code we use on this site which goes in the htaccess file:

RewriteEngine on

RewriteCond %{HTTP_HOST} ^www\.forestpathways\.co\.uk$

RewriteRule ^/?$ "http\:\/\/forestpathways\.co\.uk\/" [R=301,L]