LIRC is a package that allows you to decode and send infra-red signals of many (but not all) commonly used remote controls.
Recent linux kernels makes it possible to use some IR remote controls as regular input devices. Sometimes this makes LIRC redundant. However, LIRC offers more flexibility and functionality and is still the right tool in a lot of scenarios.
The most important part of LIRC is the lircd daemon which decodes IR signals received by the device drivers and provides the information on a socket. It also accepts commands for IR signals to be sent if the hardware supports this.
The user space applications allows you to control your computer with your remote control. You can send X11 events to applications, start programs and much more on just one button press. The possible applications are obvious: Infra-red mouse, remote control for your TV tuner card or CD-ROM, shutdown by remote, program your VCR and/or satellite tuner with your computer, etc. Using lirc on Raspberry Pie is quite popular these days.