Essentially, a mixing console accepts audio inputs, combines them and then outputs the result. For example, a mixing console in a recording studio can accept the inputs of all the devices connected to it (microphones, guitars, keyboards), combine them all into one stereo track and then output that stereo track to a CD burner. The mixing console allows a recording engineer to take numerous inputs, adjust their volumes and combine them in any way necessary.
There are two basic types of mixing consoles: analog and digital. Analog mixing consoles work with electrical signals such as those that come from a microphone. Digital mixing consoles can work with electrical signals but are also able to work with digital components, such as a computer interface. Digital consoles are becoming more popular as they become less expensive, but analog mixing consoles are still widely used.
Mixing consoles are used in almost every imaginable audio situation. Radio broadcasts, television broadcasts, recording studios, live performances and even webcasting all use mixing consoles. Large mixing consoles have dozens of inputs and outputs in order to allow the sound engineer to individually and collectively customize the sounds of numerous sources. A small mixing console might be used for amplifying a single performer by mixing a guitar signal with a microphone signal and then outputting the audio to speakers.
Mixing consoles generally have audio inputs, controls to customize audio signals and outputs for final audio. Many also have the ability to accept an audio signal, send the signal out to an external device (such as an effects unit), accept the return of the altered audio signal and then output it. This is known as an "auxiliary send." For example, to record a vocalist, a mixing console might be used to accept the audio signal from the vocalist's microphone, send it to an external effects unit that applies reverb to it, accept the newly altered audio signal and then send it to a master channel in which the singer's voice (with newly applied reverb) is combined with the background music. The final product can then be sent to connected speakers or recording devices.
EQ stands for "equalizer." An equalizer allows a sound engineer to adjust the low, mid and high range frequencies of an audio signal. For example, an engineer can take the audio signal from the vocalist's microphone and remove a portion of the signal that is high frequency by adjusting the EQ. The new signal can then be mixed with the background music. Mixing consoles generally have an EQ for each channel, meaning that separate sound sources can receive separate EQ treatment. This provides a powerful and versatile tool for controlling the overall sound of the final product.