Picture phpMyAdmin, but for metrics and time series data. You point it to a time series database (such as Graphite or InfluxDB) and the app draws the charts for you with whatever settings you need, and puts them together in pretty dashboards for easy consumption.

Its a dashboard and graph composer for realtime and and historical metric analytic (mostly via rich graphs). Usually used for monitoring or application metrics analytic but can be used in many other scenarios as well.

Graphite + Grafana are like Cacti or Munin, but orders of magnitude smarter and more powerful.

Grafana is the visualization part of the combo. Graphite is the data storage and query engine.