JRebel allows you to change code and deploy it, without having to restart the application server (such as Tomcat).This significantly reduces development time, as code changes can be "hot deployed", without the need to restart the application server. This complements a Test Driven Development approach, as changes can be made to code, tests run, and the changes verified, without the need for time-consuming restarts.
There are two versions of this tutorial, the first looks at using JRebel where you prefer a command line approach to building and running your project. The second looks at using JRebel with the Eclipse IDE. Note that you can actually use JRebel in both scenarios, so you can use the Alfresco SDK and JRebel both on the command line and in Eclipse with the same project.