Overview
Consumer electronic devices concerned with audio (for example Sound cards) often have a connector labeled "line in" and/or "line out." Line out provides an audio signal output and line in receives a signal input. The line in/out connections on a consumer-oriented computer sound card are unbalanced, with a 3.5 mm (1/8") 3-conductor TRS minijack connector providing ground, left channel, and right channel. The connections on most other consumer equipment use RCA jacks. Professional equipment commonly uses balanced connections on 6.5 mm (1/4") phone jacks or XLR connectors. In most cases changing the volume setting on the source equipment does not vary the strength of the line out signal.
Read more about this topic: Line Level