Hadoop can run in 3 different modes.
1. Standalone(Local) Mode
By default, Hadoop is configured to run in a no distributed mode. It runs as a single Java process. Instead of HDFS, this mode utilizes the local file system. This mode useful for debugging and there isn't any need to configure core-site.xml, hdfs-site.xml, mapred-site.xml, masters & slaves. Stand-alone mode is usually the fastest mode in Hadoop.
2. Pseudo-Distributed Mode(Single node)
Hadoop can also run on a single node in a Pseudo Distributed mode. In this mode, each daemon runs on separate java process. In this mode custom configuration is required( core-site.xml, hdfs-site.xml, mapred-site.xml ). Here HDFS is utilized for input and output. This mode of deployment is useful for testing and debugging purposes.
3. Fully Distributed Mode
This is the production mode of Hadoop. In this mode typically one machine in the cluster is designated as NameNode and another as Resource Manager exclusively. These are masters. All other nodes act as Data Node and Node Manager. These are the slaves. Configuration parameters and environment need to specified for Hadoop Daemons.
This mode offers fully distributed computing capability, reliability, fault tolerance and scalability.
Hope this helps