Cacti is an open source software that uses RRDTool to graph and store time-series data from data sources like SNMP. It stores data in a MySQL database and uses PHP to provide a frontend interface for creating graphs, templates, and managing users. Cacti supports unlimited graph items, auto-padding, custom data gathering scripts, and SNMP to monitor network traffic and system metrics over time through graphs. It also provides features like data source templates, host templates, and user management to scale monitoring of large networks.