It's an analytics platform for big data. Basically, you upload lots of JSON, use the custom language (http://quirrel-lang.org/) for analytical queries and the platform will handle efficient storage and querying of your data.
There are also has extras like a library for charting and reporting query responses using JavaScript and an IDE for writing Quirrel and uploading data.
The Quirrel language is much more high level than the API for ElasticSearch. It comes with things like machine learning (e.g. clustering and linear regression):
http://precog.com/developers/how-tos/machine-learning/
Even without libraries like machine learning, you can write somewhat readable queries which do some impressive things: