Alfresco is based on...

[fedotxxl]

Hello
What technologies should I know to make extensions for Alfresco platform? What is it based on?

[mrogers]

It depends on what sort of "extensions" you want to write. 

Here's a starting point.

For the Share JavaScript, XML, Freemarker.
For Explorer JSF,  Java.
For Core Repo enhancements Java, Various Scripting languages, XML, JBPM.