Avestan is an Indo-Iranic language which existed as Old Avestan in the 2nd millenium BCE and as Younger Avestan spoken during the first millenium BCE.
The languages are known from their use as the language of Zoroastrian scripture which was passed down orally, being memorized by Zoroastrian priests and recited by rote memory.
The Avestan script was not developed until the 3rd or 4th century AD, long after the language ceased to be commonly spoken. The Avestan alphabet is also known by the name Din Dabireh, which means religious writing.
The script has its origins in the older Pahlavi script. Today, many Zoroastrians write and read Avestan using the Gujarati script, but there is some renewed interest in the Avestan script.
The Avestan alphabet has 37 consonants and 16 vowels, and is written right-to-left. You can use this app to study each letter, as well as numbers, common ligatures, and punctuation, and quiz yourself on them.