HADOOP - Writing YARN applications
Writing a native YARN apps may be a little awkward because Hadoop YARN project doesn't have a higher level framework to help with this. However there are few projects which are doing the heavy lifting so that user would not have a need to worry about this boilerplate code.
Spring YARN as part of Spring for Apache Hadoop with YARN Samples
In a link you posted, yes there are two main classes, one for running the client which deploys and starts the app on YARN, other for appmaster.
You could use provided examples in jar file located at $Yarn_Home/share/hadoop/mapreduce/hadoop-mapreduce-examples-0.23.1.jar.
example:
yarn jar $Yarn_Home/share/hadoop/mapreduce/hadoop-mapreduce-examples-0.23.1.jar wordcount /in /out
/in and /out are located in hdfs. You need to put some text files in /in and /out must not exist before.
Check this for reference:http://hortonworks.com/blog/introducing-apache-hadoop-yarn/