This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Hadoop's computational tier, MapReduce, is definitely a different programming/processing paradigm from your traditional procedural/OO multi-threaded programming. It excels at processing and joining together large volumes of data in parallel. Its programming model, which is designed to minimize constructs which can lead to performance issues in distributed systems such as distributed locking, requires you to represent your code in map and reduce functions, allowing you to filter, project, aggregate and join your data together at very fast rates.
If you're new to Hadoop then I'd recommend reading "Hadoop in Action" which provides a good background on Hadoop, and gives you a leg-up on useful Hadoop use cases.