Errai requires a JDK version 6 or higher and depends on Apache Maven to build and run the examples, and for leveraging the quickstart utilities.
Apache Maven: http://maven.apache.org/download.html
Please note, that when launching maven the first time on your machine, it will fetch all dependencies from a central repository. This may take a while, because it includes downloading large binaries like GWT SDK. However, subsequent builds are not required to go through this step and will be much faster.