The letters a, e, i, o, and u are vowels. All other letters are consonants. Vowels are categorized as either long or short. The “a” in gate is long while the “a” in bat is short.
The letters α, ε, η, ι, ο, υ, and ω are vowels.
ε and ο are always short.
η and ω are always long.
α and ι can be either.
Study this first or this video. This video shows how to put the consonants and vowels together so that you can read the text. Vowels are typically classified in terms of how they sound relative to their English equivalents; A class, E class, I class, O class, and U class vowels.
The long vowels are the hiriq-yod, shureq, sere, sere-yod, and qamats; see here.
Hebrew also has half vowels as opposed to full vowels. Half-vowels are also called "very short vowels" or "reduced vowels" or "composite shevas". Any vowel with a sheva in it is a half-vowel, such as:
בֲּ hataf-patah
בֱּ hataf-segol
בֳּ hataf-qamats
Gutturals prefer these kind of vowels to a simple sheva. A full vowel is any vowel that does not have a sheva in it.
Hebrew also has vowel letters.
If you like, you can study the history of the Hebrew vowel system.