1. Frequency
Frequency is the physical equivalent of what we call pitch. It is a major factor to all music and radically affects how all things sound. The main reason why a song sounds like a song is because of the range of pitches - Bass Drum, Bass guitar, Rhythm Guitar, Vocals, Lead Guitar. Therefore it is important to understand what pitch is. Generally speaking, to cause a pleasing sound, the frequency response of the material must be equally balanced over the whole audible range. If you decide to emphasis high frequencies, the same must be done for low frequencies to counteract this.
The diagram below displays a sine wave during one period of sound (one cycle). During this time the wave raises to a positive maximum level, then drops back to zero before falling to a minimum level and returning to zero. One cycle per second is more commonly referred to as one hertz.
The number of cycles per second determines the pitch. The more cycles, the higher the pitch, and vice versa. For example, if you make the oscillator play at a frequency of 440hz, there will be 440 cycles generated per second. Doubling the pitch of 440hz (Concert A) will be 880hz. Conversely, half the pitch of 440hz is 220hz, an octave lower. Pitch can be adjusted in a number of ways. For example, the pitch shift button on a synthesizer or the modulation on a keyboard. Playing the actual note higher or lower will of course result in a change of pitch.