In order to set the context for the amount of data we are talking about, Big data is something of the order of few exabytes or even more. An exabyte is 1 million TB or 1 billion GB, ie., 10^18 bytes.
Big Data is the term used to represent the complexity involed while dealing with data of the order of exabytes, where RDBMS, such as Oracle etc., can not deal with such a large data. Big Data is about analysing the huge data - for various reasons - business informatics, a search engine, health care, employment projection etc. Big Data is generally used in predicting using existing patterns and decision making.
Big data requires worlds best technolgies to effectively process the large amounts of data, data mining technology is the desirable techlogy to deal with such large data.