In order to make openHAB act intelligent you can use rules and scripts to enhance your environment with ease and comfort.
openHAB has a highly integrated, lightweight but yet powerful rule engine included. Rules are written in a java-like language. Rules can be triggered with different types of events, like changes of certain device status change, time or system events.
Apart from defining flexible logic openHAB rules provide a set of different actions which can be taken by the rule, like sending commands and updates to devices, sending tweets or e-mails and so on.
Scripts are a way to create reusable components for your rules. If you need to use the same code from time to time in different rules, you can create a script and then reuse it in different parts of your logic.
You can find a detailed explaination on trigger events on the rules page.
Actions are predefined Java methods that are automatically statically imported and can be used within scripts and rules to execute openHAB specific operations. They can be used to send e-mails and do other kinds of notifications as well as other common things.