It’s is considered good practice, for both SEO and users, to have your website resolve to one URL. Technically speaking www.domain.com is a subdomain of domain.com. Users often type www. prior to the domain in the address bar. So to keep that formality consistent with your users you can redirect the traffic on domain.com to www.domain.com.
.htaccess
If your website is hosted on an apache server, this redirect from non www to www is a simple implementation. Add this to your .htaccess file.
RewriteEngine On RewriteCond %{HTTP_HOST} ^domain\.com [NC] RewriteRule (.*) http://www.domain.com/$1 [L,R=301]
Add the above code to you .htaccess file and replace domain.com with your domain name.
I have another post on how to redirect www to non-www with .htaccess.
Want to learn more about apache? Try this resource...