MapReduce is a programming model used for processing large data sets across distributed systems. It simplifies data processing by breaking it down into two main functions: Map and Reduce. The Map function takes input data and transforms it into a set of key-value pairs, while the Reduce function aggregates these pairs to produce a final output. This model is particularly useful in big data applications, allowing for efficient parallel processing. It was popularized by Google and is implemented in various frameworks, such as Apache Hadoop, enabling developers to handle vast amounts of data easily and effectively.