Understanding the Audio Spectrum
You probably already know that sound is made of a spectrum. The limits of human hearing in this spectrum run from about 40 Hz at the lower end to about 16,000 Hz at the upper end. At the lower and higher extremes the sound may actually be felt rather than heard, but we don't need to go into that just now.
You might already know this, but do you know how to make use of this knowledge so as to make it work for you in your mixing? The chances are you may never even have thought about it.
Different regions of this spectrum exhibit different characteristics. There's no absolutely arbitrary way of identifying precise boundaries, but the categories shown below are generally agreed to be there or thereabouts. Along with each category is listed its main characteristics.
Bass: from about 40 Hz to 200 Hz: Booming, full, solid
Low Mids: from about 200 Hz to 800 Hz: Body, fatness, fullness, warmth
Mids: from about 800 Hz to 5,000 Hz: Clear, present, forward
Highs: from about 5,000 Hz to 8,000 Hz: Bright, alive, brilliant
Ultra Highs: above about 8,000 Hz: Crisp, radiant sparkling
The way in which the sounds produced by different instruments use different permutations of these frequencies is one of the aspects which determines the sound of the instrument. One issue that you will need to address in mixing is how to prevent your different instruments (and voices) from fighting each other for ownership of the same frequencies.