Sounds are changes in air pressure such that the air is compressed or rarefied periodically as the sound waves travel. The number of compressed or rarefied patches of air that are received by our ears each second is called the frequency (measured in hertz, Hz).
Since all sound waves propagate at the same speed (343 m/sec), high-frequency sound waves have more compressions and rarefactions packed into the same region than low-frequency waves. A sound of high frequency is perceived as high tone whereas low-frequency sound is perceived as a low tone.
Sound intensity is another important feature perceived by our ears. It is the pressure difference between compressed and rarefied patches of air measured in decibels (dB). The loudness we perceive is determined by the intensity of a sound.
Sounds in the real world hardly consist of simple periodic sound waves but are in fact combination of different intensities and frequencies.