HAL is a hardware abstraction layer and software project that allows desktop applications on an operating system to readily access hardware information so that they can locate and use such hardware regardless of bus or device type. In this way a desktop GUI can present all resources to its user in a seamless and uniform manner.

Dual-licensed under both the GNU General Public License and the Academic Free License, HAL is free software.

HAL can gather information about removable storage devices and trigger their representation within the user's desktop environment.

Traditionally, desktop applications discovered hardware by communicating directly to the kernel, which maintains the list of devices connected to the system. This demanding process is not always accurate because sometimes the kernel doesn't know everything about a device. For example, some MP3 players or digital cameras show up as only another hard disk in the user interface. So not many user interfaces have been built for hardware discovery.

With HAL, all the important information about certain classes of hardware is made accessible in a uniform format. When a new device is added to the system, an asynchronous signal is broadcast on the system message bus detailing what kind of device was added. Any desktop application can connect to the message bus to discover the hardware. System-level scripts can also be run to configure the device. In effect, HAL allows plug-and-play.

The HAL daemon maintains a list of devices, that contains well-defined key or value pairs describing what the object represents. Each device object is identified by an Unique Device Identifier, or UDI. The key or value pairs (namely device properties) are typed and defined in the HAL specification, so users of HAL know what values each property means.