Hadoop
In plain terms
A framework for distributed storage and processing of big data.
Going deeper
Hadoop stores huge datasets across clusters (HDFS) and processes them with MapReduce, pioneering affordable big-data computing. It scaled data processing on commodity hardware.
Under the hood
HDFS plus YARN and MapReduce; largely superseded by Spark and cloud warehouses for processing.
Examples
Big data storage, HDFS, MapReduce, data lakes, legacy big-data systems.
Think of it like...
Like a vast warehouse network plus a crew that processes goods across many sites.