The Hadoop Distributed File System (HDFS) is one of many different components and projects contained within the community Hadoop™ ecosystem. The Apache Hadoop project defines HDFS as: “the primary storage system used by Hadoop applications. HDFS creates multiple replicas of data blocks and distributes them on compute nodes throughout a cluster to enable reliable, extremely rapid computations.”
via Comparing the Hadoop File System (HDFS) with the Cassandra File System (CFS) : DataStax.