Audio interfaces are dedicated devices with the core function of transferring analogue audio streams to and from digital computer systems. Use for questions where the audio interface is an important aspect. See the full tag wiki for more info.
Questions related to audio interfaces - e.g. Digital to Analogue and Analogue to Digital conversion questions. These devices are either internal (PCI/PCIX) or external: USB, Thunderbolt, FireWire.
Hardware engineering design is off-topic for this tag.
As always, shopping recommendations are off-topic.
The Audio Interface
The audio interface is an important piece of kit in digital and hybrid studios of all sizes. It's important because it contains A/D converters which convert incoming analogue signals to the digital form that it's recorded in the digital environment, and vice versa for playback, which can happen simultaneously.
Audio interfaces range from cheap and simple single-input/output devices to multiple inputs and outputs of different level and type. Bear in mind, consumer-level audio interfaces like, for example, Sound Blaster sound cards, are off-topic. Studio-grade audio interfaces often come with custom ASIO drivers and accompanying 'mixer' routing and settings software to utilize the full potential of the device's DSP capabilities.