In Lisp, a map takes as input a function and a sequence of values and applies the function to each value in the sequence.
A reduce takes as input a sequence of elements and combines all the elements using a binary operation (for example, it can use “+” to sum all the elements in the sequence).

MapReduce, inspired by these concepts, was developed as a method for writing processing algorithms for large amounts of raw data.


http://www.developerzen.com/2009/05/06/introduction-to-mapreduce-for-net-developers/