Just something I've wondered. In practice they obviously usually resolve to the same place but once registered and in use, what is the essential technical difference, if any, between the use of http:// and www. or even http://www. placed before a domain?

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AFAIK technically nothing. its more of a religious use

Really, its just setup with A records / CNAME

I don't know much about the details of the "set up" with CNAME or A records. Perhaps someone could answer in simple but detailed terms, is it that historically http:// was established before www protocols so they end up being used side by side?

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http is the protocol. www is not a protocol, it's just a subdomain which was widely accepted and used.

http stands for Hypertext Transfer Protocol and as the name says it's the protocol, www is a subdomain and stands for world wide web, as others said, it's just a subdomain. Historically everybody uses www and people type a webaddress with www, but there is no technical reason to use www. Always assure that your hosting company sets up the domain to work with and without www.