http and www are two different things. http is the protocol to transfer hypertext requests and information between servers and browsers. http got nothing to do with seo. however www got something with seo. unless you restrict your web pages could be served with both www and without www (provided your dns is set that way). using one version and making a redirect from another version makes sure only one version of your pages gets credit from search engines.
The difference is explained above. Better use both though. Its much convenient than choosing one. By default, this is how it should be done.
There is no difference in having your site http://example.com vs http://www.example.com However, you do not want to use both, as the search engines will see them as two different sites. Read this page. http://www.mattcutts.com/blog/seo-advice-url-canonicalization/ Pick one (either http or www) and set up a 301 redirect for the other.
If you are asking for SEO point of View then Yes, Search engines do consider site with www and without www (means just with http) differently. The solution is, one should redirect it to one address: For example if you have abc.com then once you type it just with http it should automatically redirect to site with http://www And still if the problem is there with any site, It is called Canonical Issue. Hope this may help you.
http denoted the type of connection you are making with domain - Secured - Normal www- It is a part of domain which sets the path to the domain- viz, now days you can simply use the domain itself...
Http is a protocol, while WWW is short for world wide web. You can get to a site by www domain or non-www domain. Each one is OK. But for SEO, I think you can redirect one to the other.
http means Hyper Text Transfer Protocol. WWW means world wide web. There is no relationship between this. http is the way by which data is transferred. There is another protocol also available like ftp(file transfer protocol), https(http secure), etc. WWW is the domain name. www.sample.com. If you want to remove www means sample.com, it is ok and you can do it very easily and no difference in doing seo.While http or any one protocol is must needed. You can't remove it.