The bitcoinj library is a Java implementation of the Bitcoin protocol, which allows it to maintain a wallet and send/receive transactions without needing a local copy of Bitcoin Core. It comes with full documentation and some example apps showing how to use it.
To get started, it is best to have the latest JDK and Gradle installed. The HEAD of the `master` branch contains the latest development code and various production releases are provided on feature branches.
Alternatively, just import the project using your IDE. [IntelliJ](http://www.jetbrains.com/idea/download/) has Gradle integration built-in and has a free Community Edition. Simply use `File | New | Project from Existing Sources` and locate the `build.gradle` in the root of the cloned project source tree.