Is there any difference between www.abc.com and abc.com?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Aman Tandon: I have a Website and  ;say its link is abc.com. ;
  So is there any difference between
  www.abc.com and abc.com ?
• 
  Jim Munro: Interested to read what others will have to say but I think abc.com is a subdomain just like any other subdomain although some people will dispute that opinion. It's sometimes also called the naked, bare, or root domain but it's not treated any differently to xyz.abc.com or www.abc.com. Sometimes it is useful to leave off the www if the root domain is especially long but I think it's just a matter of preference.
• 
  Jason Mathes: +Jim Munro I always thought that a sub domain had to be structured as xyz.and.com?
  
  Hmm time to do some homework!
• 
  Jim Munro: You are probably right, +Jason Mathes ;but I am regarding abc as a subdomain of the .com tld. I've never been able to get many people to agree with me. :)
• 
  Jason Mathes: True. I'd go with that's the root domain. The .com is just the domain type? Like I said time to do some homework!
• 
  Aman Tandon: I am a little confused . See my website domain is Hackbook(dot)biz but when people type Hackbook(dot)biz they get redirected to www(dot)hackbook(dot)biz . So is there any difference between them ?
• 
  Jim Munro: Whoever set your website up, did the right thing. You do not want both the www and non-www versions of your site competing for the index so the non-www version is 301 redirected to the www. version.
  
  BTW please rewrite your links above to www.hackbook(dot)biz, Aman. I am sorry to be pedantic but it is one of our rules here. Thank you for your cooperation and understanding.
• 
  Aman Tandon: +Jim Munro ;thanks for your Answer . Yeah I was unaware of the rules so thanks for informing this too . ;
  
  In one of your hangouts with Dejan Seo you all said that there is indeed a difference between www version of a website and domain(dot)com version .
  
  And I set my website up  ;:-D .
• 
  Jim Munro: You have not done yourself a disservice though as most people will naturally link to you as www and a smaller proportion will expect to see a www. I'd like to watch that clip again, can you point out which one it was?
• 
  Aman Tandon: +Jim Munro Here is the clip where you all were analysing websites . And one of them had a domain like domain(Dot)com and your comments were unclear to me . SEO Q&A: Bloggers
• 
  Jim Munro: Thank you for taking the trouble. Aman. ;
• 
  Sandy Ng: Yes, SE treat them as 2 sites. You have to redirect your site abc.com to www.abc.com using 301 redirect, so the incoming links to abc.com will send to www.abc.com instead. ;
  
  Hope this help
• 
  Aman Tandon: thanks it really helps :-) ;