Old High Gallifreyan is one of the dialects of the Gallifreyan language. It stopped being used during Rassilon's reign over Gallifrey, and few Time Lords could understand it. The Doctor could understand it, and apparently River Song also could understand it. The Worshipful and Ancient Law of Gallifrey is written in Old High Gallifreyan.
The language looked like that of Ancient Greek (it's speculated that a renegade time lord implanted aspects of the alphabet into Ancient Greece), and consisted of a standardized alphabet, originally made up of approximately 60 characters.
I'm currently designing a Old High Gallifreyan font, it will be slightly modified, using the 26-character format, but I plan to make a 55-character format eventually.