Apache Storm, in simple terms, is a distributed framework for real time processing of Big Data like Apache Hadoop is a distributed framework for batch processing. Apache Storm works on task parallelism principle where in the same code is executed on multiple nodes with different input data.
Apache Storm does not have any state managing capabilities. It instead utilizes Apache ZooKeeper to manage its cluster state such as message acknowledgements, processing status etc. This enables Storm to start right from where it left even after the restart.