Alfresco - /alfresco and /share difference
/alfresco
This context points you to the Repository application. It's the "core" app, where the content is effectively stored and where all the magic happens. Back in the old times, it was the one and only application, so that when Alfresco introduced Share, it was the only place where to find a UI for some features. Now, Share is missing just a couple things.
Currently, the idea is that this JSF application should be a kind of Admin only interface, only to be accessed whenever Share is missing something. However, if you really love JSF (really?) you can still opt to use this interface to build your Alfresco implementation.
This application is mainly interesting for its features rather than for its UI: all the connections to external systems, like DB, CIFS or IMAP for example, happen from here.
On a side note, the "original" implementation of Alfresco WCM is built around this application. Anyway, because of several reasons this implementation of WCM is being phased out, with a new implementation built around Share and its concepts (e.g. Sites) that will grow quite a lot soon.
Don't expect new fancy development on this UI.
/share
This is usually intended the main UI for end users. No JSF here (thanks, God!), it's a Spring Surf frontend to Alfresco that focuses on providing a Collaborative environment. Here you will find the concepts of Sites, project/topic focussed containers for content, blogs and wikis.
As all the content is stored via the other application, Share is almost stateless: no DB connection is opened from here, everything is pulled and pushed to /alfresco via Alfresco REST API and CMIS.
As already mentioned, the new WCM is leveraged from here, as well as all the common ECM features of Alfresco such as workflows, versioning, document transformations, ACLs and so on. Plus, document previews and thumbnails, Blogs, Wikis, Data Lists, ...
This is most likely the UI Alfresco will continue to improve, adding all the whistles and bells they can come up with.
Adding some more information on latest version alfresco 5.0b.
Major changes in alfresco 5.0b is that Alfresco web client is removed from alfresco.So there wont be any JSF coding
Repository Architecture is still there, So if you want to deal with core app "/alfresco" is the location which will be used for creating webscript,workflows and other thing.
Conclusion
- Share will be only used as frontend application.All core portion will be on alfresco repository.
Alfresco: By a developer point of view I would describe "alfresco" as the backend repository that sums up most of the functionality for which we use ACS. The services that share uses are defined in alfresco.
Share: Share was introduced so that a person with not so much technical knowledge may be able to get as much as possible out of the ECM. If you are new to share you should look into model manager, smart folders and actions.