In this paper, we describe Big Data and open source framework Hadoop. The statistics are provided, explaining formation of Big Data and the importance of analyzing it. Big Data possess many problems in the real world scenario due to it’s vast size, velocity and variety. Several techniques are developed to process Big Data. Hadoop framework is one such technique and is described along with the pseudo code for its mapper and reducer function. The file system of Hadoop – HDFS is explained as well with the help of appropriate diagram.