There is a lot of not-quite-correct information about Egyptian hieroglyphs in the replies to this comment. Hieroglyphs are actually a hybrid system, incorporating uniliteral glyphs (representing a single consonant sound), bi- and triliteral glyphs (representing two or three consonants), and logograms, representing a full word or concept. They also include "determinatives," which are basically helpful pictures at the end of a word that tells you something about the category of the word. Sometimes, a word incorporates all three types of symbol. For example, the word "Ra," if it refers to the sun god, might have the two consonants r and ' (which is close to a), followed by a picture of the sun, followed by the seated god determinative.

So I guess the real answer here is that the Egyptian writing system incorporates elements of alphabets and logographs, used together. Vowels were (mostly) not marked, though some "consonants" are treated as vowels in transliteration. I'm not entirely sure what the distinction is there.