Imagine a gigantic tsunami. But instead of water, substitute it with a tidal wave of data. That should give you a pretty graphic idea about the enormity of big data.
Big data is characterized by volume, velocity, and variety.
Volume refers to really LARGE quantities of data. Think petabytes, where each petabyte contains a million gigabytes.
Velocity refers to the speed with which data is gathered, such as what social media can generate daily.
Finally, variety refers to the different formats — i.e., text files, audio and video files, email, spreadsheets, etc. — in which the data arrives.