The function of audio hardware is to transfer audio signals in and out of your PC. A huge range or audio hardware is available to PC users. Even the simplest audio hardware can be used to create high-quality audio, as long as the audio doesn't leave the digital domain. However, a professional audio interface will allow more flexibility, better sound monitoring, and better support for high-quality recording.
Professional audio interfaces are optimized for real time (or at least low latency) multichannel sound recording and playback, and studio-grade fidelity. They often have an assortment of analog and digital connectors and may provide built-in preamps for microphones. Their drivers usually follow the Audio Stream Input Output (ASIO) protocol for use with professional sound engineering and music software, although ASIO drivers are also available for a range of consumer-grade soundcards.
Professional audio interfaces often have the form of external rack-mountable units using USB 2.0, Firewire, or an optical interface. Their role and intended purpose is similar to a specialized multi-channel data recorder and real-time audio mixer and processor, roles which are possible only to a limited degree with typical consumer soundcards.
On the other hand, certain features of consumer soundcards such as support for Environmental audio extensions, optimization for hardware acceleration in video games, or real-time ambience effects are secondary or nonexistent in professional audio interfaces.
The preceeding takes material from the following article:
Wikipedia contributors, "Sound card," Wikipedia, The Free Encyclopedia (accessed December 24, 2008).