Latency
From WikiRecording
Latency is the amount of delay (in milliseconds) for a DAW to process and output a signal. Latency values of greater than several milliseconds can make monitoring through the DAW impractical, as the delay will be perceptible.
Latency can be introduced in a few ways. In mixing it can sometimes be introduced by adding Real-Time Effects. The latency is usually relatively small, and only a few milliseconds. If it is perceptible you can shift the audio forward to counter the delay, or write the effect to a new track removing the plugins latency.
Another common latency problem is the buffer setting in your DAW that can sometimes be a problem when monitoring your input.
Here is a simple formula to approximate buffer latency:
Latency can be improved by reducing the buffer size in your audio card's settings application, but too low of a buffer size may cause the program to report an error, or include artifacts such as clicks, pops, and dropouts in your audio. Only experimentation with your particular hardware can determine the best tradeoff between buffer size and latency.
Most DAW, and Tape machines have a way to monitor without latency, on analog tape this is usually achieved by not listening from the playback head but instead listening on the record head, usually adjusted by setting the machine to "sync". On a DAW there should be a way to monitor the live input signal before it's run into the computer and sent back out, thus avoiding the latency. For example on an Mbox the "mix knob" when positioned upright allows you to monitor and listen back to your mix, just make sure to mute the track your recording on so it's not doubled.
