Alfresco Share is one of the one part of the Alfresco as product- it's user interface on the Alfresco repository (platform), by default it is avaiable on <host>:<port>/share
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It is built on the Surf framework. The Surf framework was developed by Alfresco, but in 2009 Alfresco began working with Spring Source and announced the Spring Surf Extension framework. Later on, both Spring Source and Alfresco were collectively developed and are available as plugins in Spring MVC 3.x.
Refer to the following links for more details:
It simplifies working with documents as a saving them, sharing, and making basic Alfresco features with it- eg. changing content type, filling properties, or running a workflow processes on them.