Why is the English alphabet divided into vowels and consonants? Why are they considered different from each other?

What is a vowel?
A vowel sound is pronounced with the mouth open and allows the air to flow freely through it from the lungs.

When the doctor tells you to open your mouth and say Aaah... You can open your mouth wider, move your tongue in the mouth (without touching another part of your mouth) and move your jaw up and down and it will produce the A sound. You have not restricted the movement of air from your lungs through your mouth so it is a vowel.

The following letters represent a vowel sound: A E I O U (and sometimes Y)

What is a consonant?
A consonant sound is pronounced with some type of constriction of the flow of air that involves either the lips or the tongue.

For example with the consonant P. Just before you pronounce the letter P, notice how your lips are pressed together stopping the air from coming out of your mouth. When you quickly release your lips, it produces the P sound. This constriction means you are producing a consonant sound.

A consonant requires the mouth and tongue to "shape" the sound.

The following letters represent a consonant sound: B C D F G H J K L M N P Q R S T V W X Y Z