Alfresco is ECM (enterprise content management) system, based on java frameworks and javascript. Alfresco (as a "repository" is the base core of the Alfresco as a product).
It provides eg. encrypted content store, specific permissions settings, content types with specific properties (like name, date of creation, also custom properties can be included).
Alfresco also provides Workflow processes- separately or with documents, workflows are specified by diagram flow (Activiti and jbpm engine is supported; jbpm deprecated).
Product also partly supports Sharepoint (via Alfresco office services- "aos", CIFS, and other features. Key features are:
User content is wrapped with the security layer- binary content is saved on the disk, but the structure and metadata are saved in the database.
This part- "repository" of the Alfresco as product provides REST API for the extensions like Alfresco Share.
Alfresco Installation Link For Windows
http://docs.alfresco.com/community/tasks/simpleinstall-community-win.html
For Linux
http://docs.alfresco.com/community/tasks/simpleinstall-community-lin.html
Brief Description about Installation
http://docs.alfresco.com/community/concepts/install-community-intro.html