REST is a set of ideas that promise to simplify communication between applications.
If you have not come across REST before, there is an entertaining description here.
Conventionally applications use something like Remote Procedure Calls to call on each other for services, but every single call tends to have different parameters and return values making it hard to generalise and fiddly to use. In REST, the interesting parts of an application are available as ‘resources’. Alfresco offers resources such as spaces (folders), files, people, tasks etc. Each resource can only do a strictly limited number of things, it can be created, deleted, changed or just read. That’s it. (The technical success of the World Wide Web can be largely attributed to REST, the resources are things with URLs, pages, pictures, tables of data, music etc. The operations in HTTP are PUT, POST and DELETE).
The designers of Alfresco have opened up the monolithic Java jungle inside Alfresco to expose the important resources. They have even documented it, a full list of approximately 370 Web Scripts (REST services) available from our Alfresco server is available at http://epsilon.nuhrise.org/alfresco/service/index, but you need to log in with the admin password!