Audio Stream Input/Output
From Hydrogenaudio Knowledgebase
Audio Stream Input/Output (ASIO) is a low latency audio application programming interface designed by Steinberg for high speed communications between audio hardware and software, such as the the interaction between MIDI Controllers and MIDI Sequencers needed by musicians and studio engineers. ASIO can be used to access multiple sound I/O simultanously.
 Realtime Recording/Playback
In order to create a more direct path to the hardware, and also make the hardware more responsive to the software's needs, Steinberg of Germany (developer of the popular Cubase MIDI sequencer and Nuendo digital audio workstation) developed Audio Stream In/Out (ASIO) technology. ASIO has made its mark as the driver standard for delivering low-latency transfer of digital audio. This means that hardware supporting an ASIO driver is able to deliver low-latency recording and monitoring to hard disk.
The low latency made possible by ASIO, for example, allows you to sing into a microphone and hear your voice going through software and hardware effects with virtually no delay in the signal. The effects of a low-latency ASIO driver make it ideal for use with Virtual Studio Technology (VST) instruments, plug-ins, and all other software-based synthesizers. Many of these devices can be played back in real time from MIDI output devices.
After Steinberg was bought by a seperate company, they released the SDK (Development Kit) to the public free of charge on their website. There have been several attempts made by 3rd party developers to write drivers in order to bring ASIO to all consumer grade soundcards (see asio4all).