MapReduce is a Hadoop function that is used to efficiently access Big Data stored in HDFS (Hadoop Distributed File System). MapReduce is the core module of Hadoop that effectively segments huge data sets into smaller chunks and processes them in parallel. On the other hand, Pig is a tool or platform that works as an abstraction layer over MapReduce and the Hadoop ecosystem for better processing of Big Data. While MapReduce helps distribute and store data across server networks, Pig is used much like SQL to manipulate the stored data. So, while Pig is a high-level data-flow language, MapReduce is a low-level data processing language.