As we know, musical instruments consist of a vibration which is amplified by resonance. The human singing voice is no different. The vocal chords are the vibrating part and the throat, mouth, nasal cavities and bronchial tubes constitute the resonance cavities that amplify these vibrations into sound. Because every person's combination of throat, mouth, nasal cavities and bronchial tubes is slightly different, we all sound slightly different. The fact that we can change the shape of some of these cavities at will enables us to produce a wide range of pitches, depending on the initial structure and training. Here is Wikipedia on the human voice.
Vocal cords, vocal tract, Bernoulli's principle, resonance cavities, vocal formants, trachea, larynx, phonemes (plosive, fricative, vowels, diphthongs, consonants, semivowels, gliding consonants, liquids, nasal), throat singing, articulation (opera singers).