We’re all use to using URLs to point at web pages but we too often forget that they can be use for other things too. They can address any resource and that includes: people, documents, images, services (e.g., “today’s weather report for London”), TV or Radio Programmes in fact any abstract concept or entity that can be identified, named and addressed. 
Also, because these resources can have representations which can be processed by machines (through the use of RDF, Microformats, RDFa, etc.), you can do interesting things with that information. Some of the most interesting things you can do happen when URLs identify people.
Currently people are normally identified within web apps by their email address. I guess this sort of makes sense because email addresses are unique, just about everyone has one and it means the website can contact you. But URLs are better. URLs are better because they offer the right affordance.
If you have someone’s URL then you can go to that URL and find out stuff about that person – you can assess their provenience (by reading what they’ve said about themselves, by seeing who’s in their social network via tools such as XFN, FOAF and Google’s Social Graph API), you can also discover how to contact them (or ask permission to do so).
With e-mails the affordance is all the wrong way round – if I have your email address I can send you stuff, but I can’t check to see who you are, or even if it is really you. Email addresses are for contacting people they aren’t identifiers; by conflating the two we’ve gots ourselves into trouble because email addresses aren’t very good at identifying people nor can they be shared publicly without exposing folk to spam and the like.
This is in essence the key advantage offered by OpenID which uses URLs to provide digital identifiers for people. If we then add OAuth into the mix we can do all sorts of clear things.
The OAuth protocol can be used to authenticate any request for information (for example sending the person a message), the owner of the URL/OpenID decides whether or not to grant you that privilege. This means that it doesn’t matter if someone gets hold of an URL identifier – unless the owner grants permission (on a per instance basis) they are useless – this is in contrast to what happens with Email identifiers – once I have it I can use it to contact you whether you like it or not.
Also because I can give any service a list of my friend’s URLs without worrying that their contact details will get stolen I can tip up at any web service and find which of my friends are using it without having to share their contact details. In other words by using URLs to identify people I can share my online relationships without sharing or porting my or my friend’s contact data.
You retain control over your data, but we share the relationships (the edges) within our social graph. And that’s the way it should be, after all that all it needs to be. If I have your URL I can find whatever information (email, home phone number, current location, bank details) you decide you want to make public and I can ask you nicely for more if I need it – using OAuth you can give me permission and revoke it if you want.
Photo: Point!, by a2gemma. Used under licence.
Derivadow 
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