If you are new to Alfresco, you should browse the Getting Started Tutorials in the official documentation. There are also help buttons within the applications that will take you to the correct section of the User Documentation.
If you are responsible for the administration of an Alfresco system, then you should reference the sections on Installing, Administering, Troubleshooting, and Reference. This unofficial guide to Alfresco's Maven artifacts might also be useful (the official guide to the Maven SDK is focused on development).
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:
http://docs.alfresco.com
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.
There are a number of excellent books about Alfresco. We are partial to the one our engineering team wrote. There is a curated list on the front page of the wiki.
Our training team has a complete set of courses to guide you to your Alfresco goals. You can also contact us or a Certified Alfresco Partner for support or consulting.
An up-to-date list of Alfresco resources, including a list of places you can go for help, is here:
http://www.alfresco.com/resources/documentation/getting-started/
