An abjad, also called a consonantary, is a writing system wherein the only signs are consonants, and vowels are left unwritten. (Sometimes vowels that are sometimes consonants, like y and w, are written also.)
The name "abjad" comes from two letters of the Arabic abjad.
Semitic languages, so I understand, are well suited for abjads, because their morphology is based on the manipulation of consonantal roots. Hebrew, Aramaic, and Egyptian also use or used abjads.
Some of these writing systems use diacritics called matres lectionis ("mothers of reading") to indicate vowel sounds when teaching the language, or when otherwise important (for example, in sacred writings).