Graphite is a passive metrics storage tool. The reason we call it passive lies in its inability to collect metrics. They need to be collected and pushed in a separate process.
It is a time series database with its own query language and capabilities to produce graphs. Querying API is powerful. Or, to be more precise, was considered powerful when it appeared. Today, when compared with some other tools, its query language is limiting, mainly due to its dimensionless format for storing metrics.
Graphite stores numeric data in time series format. Its metric names consist of dot-separated elements.
Data is stored on a local disk.