HAL is a hardware abstraction layer and aims to provide a live list of
devices present in the system at any point in time. HAL tries to
understand both, physical devices (such as PCI and USB) and the device
classes (such as input, net, and block) physical devices have. Besides
it allows merging of information from device info files specific to a
device. HAL provides a API through D-BUS for querying devices and
notifying when things change. Finally, HAL provides some monitoring (in
an unintrusive way) of devices. For further information, please refer
to the HAL specification.