The alphabet is split into vowels and consonants.
Here are the vowels:
a e i o u
Consonants are all of the other letters in the alphabet:
b c d f g h j k l m n p q r s t v w x y z
All words need to have at least one vowel.
You can change words into different words by changing their vowels or their consonants.
Look at this word ladder:
The word cat changes to bar by changing a consonant each time you travel down the ladder.