when registering a domain, it is not needed. it is needed for the url of a website. You may create it through you domain registrar account panel or through your hosting panel.
While registering... it is only the name and the tld which are absolutely an essential requirement! Regards, RightMan
It isnt necessarily 'needed' for the url, per se', but most humans naturally type it anyway. And I use google webmaster tools to make sure the www is always included in my indexed pages.
its not needed, you can register/use your domain without "www", as damoolag said its just a sub-domain.
You dont need the www, but you should choose whether or not you are going to use it and stay consistent. Use the version you choose on all business cards, advertising etc etc etc as your customers can become familiar with one or the other. You can nominate your preferred choice in google webmaster so your search results are consistent and as siberforum said, you can (and should) use redirect to make sure all your linkjuice is pushed through one web address.
when you add a new domain on hosting accounts, it automatically create a DNS record for WWW. so typing WWW.DOMAIN.COM is same as DOMAIN.COM All my domains are redirected to DOMAIN.COM even typing WWW.DOMAIN.COM on the url
No you don't need it. But if you're doing SEO, do not link back to your site with both the "www" and without it. You will really screw your site. Matt Cutts talks about it in one of his videos on his YouTube channel. Use one or the other.
have you ever tried to enter in digital points forum domain WITH the WWW?? doesnt work!! hXXp://forums.digitalpoint.com vs www .forums.digitalpoint.com the www. doesnt work for me. this is a first in my experiences. but im sure they have a reason.