A, E, I, O, U - those are the vowels. Everything else is a consonant. Seems simple, but English is sneaky. Sometimes Y acts as either!
A vowel counter helps you quickly analyze text. Writers use it, language learners use it, and sometimes I just use it for fun.
Beyond Just Counting
Good vowel counters show you both the count and the percentage. Interesting data point, even if you're not sure what you'd use it for!