The terms consonant, vowel and diphthong were used in referring to IPA (International Phonetic Alphabet) symbols. These terms describe different kinds of sound which are found in all languages.

1. Consonants
   
   When we speak, a stream of air is forced out of the lungs, travels upwards through the throat and then passes out through the mouth and nose.
   
   Any sound that is produced by blocking the air stream from flowing out easily through the mouth is called a consonant.
   
2. Vowels
   
   But when certain other sounds are produced such that there is no blocking of air stream in the mouth. The air stream passes out freely through the mouth and nose, such sounds are called vowels.
   
3. Diphthongs
   
   In the pronunciation of some words, two vowels  combine together to form a single sound, which is called as a diphthong.