If you are new to Alfresco, you should browse the Getting Started Tutorials in the official documentation. There is are also help buttons within the applications that will take you to the correct section of the User Documentation.
If you plan to develop solutions on top of Alfresco, then the best place to get started is the Alfresco Developer Portal. The documentation sections on Customizing, Programming, Troubleshooting, and Reference should also be helpful. Many of the add-ons in this directory provide valuable examples, especially the Share Extras project. If you need to dive deep, then the project JavaDocs will be useful.
The latest version of all of our documentation can be found at:
Volunteers have provided valuable supplemental information in the Community Wiki. Frequently referenced pages include Packaging and Deploying Extensions, Multilingual Document Support, and Language Pack Development.
An up-to-date list of Alfresco resources, including a list of places you can go for help, is here: