David Menninger writes a nice intro to Splunk in Splunk Makes Machine-Generated Big Data Serve Analytics:
Splunk focuses on a specific segment of the big-data market: machine-generated data. This type of data originates constantly from many sources throughout an organization and in large quantities. The other common characteristic of machine-generated data is that generally it is less structured than data in typical relational databases. Often the information is captured as logs consisting of text files containing various record lengths and record structures. To effectively utilize this loosely structured information in real time, two challenges must be overcome: loading the data quickly and easily navigating through and analyzing the information once it is loaded.