A sound card or audio interface usually has two types of input jacks: low signal and high signal. The low-signal input is used for microphones and other devices that produce a weak signal requiring amplification by the sound card. The jack is usually marked ``mic in'' and is often colored red.
The high-signal input is used for electronic components, such as tape decks, DVD players, VCRs, amplifiers, receivers, and mixers. A number of different labels are used for the high-signal input. Some of the most common are ``aux in'', ``line in,'' or ``audio in.''
A turntable is one of the few audio components that uses a low-signal input, but turntables are usually routed into into a preamplifier or amplifier rather than directly into the sound card. The preamplifier or amplifier, in turn, would be plugged into a mixer or directly into the high-signal input jack.
It is important not to confuse the low- and high-signal inputs. An electronic component plugged into the sensitive low-signal input will overpower it and produce distorted sound. On the other hand, a microphone plugged into the high-signal input will be barely audible.