Input levels can be either line level or mic level. Remember when plugging things in that line-levels are much "stronger" than microphone levels; if your mixing desk is expecting and microphone-level signal, and you feed it a line-level signal, one or more circuits on your desk are going to overload, resulting in distortion. While distortion in and of itself will not necessarily harm the mixing desk, if subsequent pieces of equipment are powered on and active, severe distortion in loudspeakers can be damaging, not to mention annoyingly loud. If you have to patch equipment while the equipment is on (which is not a wonderful idea), keep all level controls down.
Record players, phonographs and turntables
When records are manufactured, a special equalizer takes most of the low-frequencies out of the audio signal, and boosts a lot of the high-frequencies before the record is pressed. When the record is played back, the low-frequencies are re-equalized in, and the boosted high-frequencies are filtered down. This equalization is referred to as the RIAA Equalization Curve
Other playback gear
The outputs from popular playback gear, such as DAT (digital audio tape) decks, MiniDisc machines, Compact-Disc machines, computer-based audio workstations, and the like all have line-level outputs. Depending on the piece of gear, the outputs may be unbalanced signals or balanced signals. Unbalanced outputs usually terminate in 1/4" TS jacks or RCA (Cinch) jacks, whilst balanced signals usually terminate in three-pin XLR-type connectors or 1/4" TRS jacks.
Electronic instrument levels are comparable to line-level signals. Generally, they are unbalanced signals. If the destination, i.e. your mixing desk, is in close proximity to your keyboards, samplers, tone generators, drum machines, or other electronic devices, then there is very little reason not to take an output directly out of the instrument and connect it directly to a line input on the mixing desk. If, however, it is necessary to place the mixing desk far away from the instruments (i.e. in the case of orchestra pit to front-of-house), one should use a direct box in order to rectify impedance differences between the instrument and the mixing desk, and to take the high-impedance unbalanced signal and convert into a low-impedance microphone level balanced signal.
Direct boxes are devices that convert unbalanced level outputs into balanced, microphone-level inputs. They are used heavily in sound reinforcement, when it is necessary to drive long runs of cable between instruments and the front-of-house mixing desk. Its operation is quite simple-- take an output from the instrument in question and patch it into the "Input" on the direct box. A "Link" output of the direct box allows daisy-chaining of the instrument output into a local amplifier or mixer arrangement. The "Output" of the direct box, usually an XLR connector, is the microphone-level balanced output, which is run to the front-of-house mixer. Direct boxes can also have other options-- an internal attenuator pad switch, a low-frequency rolloffswitch, and/or a ground lift switch. Attenuator pads can aid in reducing the level at the mixing desk from a hot signal, an LF rolloff switch can aid in cleaning up low-frequency garbage, and ground lift switches can aid in rectifying potential ground loops-- if audio equipment is grounded in too many places, a loop of potential difference will form, and will induce a hum into the audio system. The ground lift switch will disconnect the shield of the microphone cable from the direct box, thereby eliminating one possible loop point..