Audio interface
The external sound card of the home studio: the box where microphones and instruments become numbers and numbers become sound in your headphones.
An audio interface bundles everything a computer's built-in sound chip does badly: microphone preamps with phantom power, instrument inputs, low-noise AD and DA converters, and often digital I/O such as ADAT or S/PDIF. The connection is usually USB, or Thunderbolt or Ethernet on pricier units.
What separates a good interface from a bad one is rarely the spec sheet and usually the driver. The number that matters is round-trip latency: how long a note takes to go in, through the DAW and back out. On Windows that means ASIO; on Mac and Linux the USB Audio Class 2.0 class driver is usually enough, so the box works without installing the vendor's software at all.