Big data generally refers to datasets which are very large (terabytes and pegabytes) which cannot be easily stored and analysed. This data is being generated from a number of different sources including: social media posts, digital pictures, videos, transactions, GPS signals, click-stream data from websites.
For a more complete definition of big data I have included two excellent sources:
Here's a link to Big Data World Australasia 2012 a conference that will further explore how big data is having an impact on business. It is being held at Atlantic Group V on November 27-28th in Melbourne, Australia.