The Ethiopic writing system, referred to in modern romanization as Gə'əz, was developed sometime during or shortly before the fourth century CE. Two scripts were in wide use in the area at the time, the Minean script and the Greek alphabet. Due to its wide use in Saudi Arabia and the linguistic similarity between the Sabean language for which the script was used and Ethiopic, Minean forms the root for the majority of Ethiopian characters. Those for which Minean had no equivalent were modeled after Greek instead. It was initially an abjad, with only characters representing consonants. The character order broke sharply from that of other Semitic languages such as Arabic and Hebrew. It is thought that Ethiopic preserved an older order that gradually gave way to the standard order of aleph, bet, etc. in other Semitic writing systems.

While an abjad is perfectly suited to Hebrew, Arabic, and other similar Semitic languages, it did not work well for Gə'əz. There were too many words that were orthographic duplicates without indicated vowels. The catalyst for change was an increased number of students of the language. A series of vowel signs were introduced to be added to the consonant base, transforming Ethiopic into an abugida in which vowels are marked by diacritics. Most of these diacritics were horizontal/vertical bars or small dots positioned in different relative portions of each character's square space. Consonants without diacritics were assigned an inherent vowel of ä (eh). The vowel sign for ə (the schwa of sun) also doubled as a nullifying character for consonant clusters. This was consistent with Ethiopian phonetics, in which the schwa was often dropped when a word was inflected.

The unsuitability of many characters for badly positioned vowel signs and handwriting traditions caused a plethora of irregular characters to evolve. The practical consequence is that modern Ethiopic is a syllabary with vague internal consistency. It does not distinguish doubled consonants or vowels, both of which are phonetically important to the language, but it is otherwise regularly phonetic.

Interestingly, when students of Ethiopic as a liturgical and historical language were taught, they first learned to read the script with correct pronunciation, stress, and intonation before actually learning the language itself. The result was that students at a certain level could read out loud fluently and proficiently, yet not have the slightest inkling what they were actually saying. Imagine the possibilities for a practical-joking pedagogue...

Source used:
Daniels, Peter T., Bright, William. The World's Writing Systems. Oxford: Oxford University Press, 1996.