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There are two core concepts behind LiveDirectory: Profiles and Subscriptions. Generally speaking, 'content publishers' (website owners, newspapers, businesses, shops, bloggers etc) would be more interested in 'Profiles' and 3rd party application developers would be more interested in 'Subscriptions'.
A 'profile' is simply a set of data. For example, here is my 'profile'. A profile can contain any data you wish. If you own a shop, you may want to create a profile for your shop that includes your 'address', 'phone number', 'opening times', 'special offers' and even your entire shop catalog. However, if you are a newspaper, you may wish to include your latest news headlines.
Why would you do all this? Very simply, so that anybody interested in your content can 'subscribe' and will be automatically and instantly notified when you update your profile. Imagine if you publish your 'special offers' in a LiveDirectory profile and a 'text translation and twitter' service subscribes to your profile - everytime you publish a new special offer that service could translate your special offers into numerous languages and notify people around the world within seconds... pretty cool huh?
A 'subscription' simply pairs up a 'profile data path' (eg: 'jquery/latest_version') with a notification URL. Whenever the content referenced by the 'profile data path' changes, LiveDirectory will perform an HTTP POST request to the specific 'notification url'.