The Greek alphabet is a set of twenty-four letters that has been used to write the Greek language since the late 9th or early 8th century BC. It was the first (and therefore oldest) alphabet to use a separate symbol for each vowel and consonant. It is still in use today.
The Greek alphabet descended from the Phoenician alphabet and in turn it gave rise to many other alphabets in Europe and the Middle East, including the Latin alphabet. It’s letters are also used today as symbols in mathematics, science and physics. Many stars are also named after letters from the Greek alphabet.

The Greek alphabet that we recognize today came into use after the Greek Dark Ages (1200 BC and 800 BC). It’s biggest change from the Phoenician alphabet on which it was based was the addition of vowel letters. The first of these were A (alpha), E (epsilon), I (iota), O (omicron) and Y (upsilon).
Originally there were several variants of the Greek alphabet, most importantly a westerna and an eastern one. The Western version was the basis for the Latin alphabet and the eastern version is the basis of the present Greek alphabet. It was originally written right to left, but over time it evolved to be written from left to right.
Athens originally used the Attic script for official documents such as laws and the works of Homer: this contained only the letters from alpha to upsilon, and used the letter eta for the sound “h”. In 403 BC Athens adopted the Ionic script as its standard and shortly thereafter the other versions disappeared.
The pottery above shows early Greek alphabet letters.
You can visit the Classical Language Instruction Project to listen to actual samples of Greek (and Latin) rpose and poetry read by various scholars.
sources: Greek-Language.com, wikipedia

On Beyond Zebra is a classic illustrator children’s book by Theodor Geisel, better known as Dr. Seuss. It was first published in 1955 by Random House Books. In typical Dr. Seuss fashion our narrator recounts the story of his young friend, Conrad Cornelius o’Donald o’Dell, who has just learned his alphabet. Like all young learners, Conrad is taught that the alphabet ends at Z. He is amazed when the narrator presents him with more letters that come after Z.
Each of the letters that Geisel introduces in the book are used to spell the name of an exotic creature. Additionally, they are all monograms, meaning they basically combine one or more other letters. For instance, the first letter introduced is “YUZZ.” It’s the letter used to spell Yuzz-a-ma-Tuzz, a wild looking Dr. Seuss animal creation. The “YUZZ” letter looks like a weird combination of Y and Z. The “WUM” (for Wumbus) is a combination of a W and an M.
Apart from the simple whimsicalness of the book, Geisel presents us with a far deeper message. As his narrator says at the beginning, “You can stop, if you want, with the Z, Because most people stop with the Z, But not me!” Geisel is encouraging us all, both young and old, to go beyond the boundaries set for us by conventional society. Learning is a never ending process that requires constant effort and wide exploration. By using something like the alphabet, which clearly has a beginning and and end, and inventing more letters he challenges us to look beyond what we think we know. It is, of course, a great message. Many, many things have been discovered and/or invented after all of us were told quite simply that it couldn’t be done. By pushing past limits and boundaries of what we think we know, what we think we can do, what we think we understand, we can take ourselves (and possibly society) to new places. The story is a parable to remind us just how much we don’t know (even when we think we know all there is to know about something) and just how much more there is out there for us to discover.
There is actually an “unofficial” Unicode Seussain Latin Extension (U+E630 – U+E64F) for the letters which Geisel invented!