If you have ever wondered what Hahoop, Memcached, NoSQL, Mapreduce or many of the other data based technologies that are springing up are, this is a superb whistle stop tour or the market.
Hadroop is basically a system for distributing your data across multiple storage servers that all have processing capacity. Then when you want to find out something about your data, you formulate your question and it is passed to each server which then interrogate the data that it stores.
One key point from the video is that Big Data and RDBMS technologies operate on different problems. RDBMS systems are very well suited for problems requiring OLTP and data warehouse type solutions and Big Data systems are never going to encroach on this area. Big data addresses a new problem area of unstructured data and the questions that it poses. They are 2 complementary rather than competing technologies.
Highly recommended viewing.