Apcupsd is the free software equivalent of the APC's proprietary software called Powerchute. As of version 3.14.0, Apcupsd has support for the PowerChute Network Shutdown function as well as many other features.
Apcupsd runs in daemon mode so to keep a live connection with the UPS. Depending on the settings and type of connection, Apcupsd either polls the UPS to learn about its current state, or receives messages from the UPS itself, e.g. via SNMP traps. Possible types of connections to the UPS are USB, RS232 or Ethernet. Apcupsd can communicate with other instances of Apcupsd on other computers and maintain a client-server relationship with them. This way it is possible, to power multiple computers with one UPS, although only one of them is connected to the data port of the UPS.