Basically, sounds are divided into two parts: vowels and consonants.
Vowels are produced by letting the air flow through the mouth without disturbing it. Vowel sounds are “a, e, I, o, and u”. While speaking these letters, you can notice that your mouth is open such that the air flow is clean. On the other hand, consonants are the sound created by breaking the air flow. Any letter except a, e, I, o, u is a consonant for it breaks the air flow while speaking. For instance, try speaking letter c or x. you will notice that while speaking these letter the air flow is broken.