From the introduction:
Realms of Adventure...is set in the medieval era, with castles, magic, and fantastic creatures. You can play a knight, a wizard, a rogue, a crusading priest, or any other character you chose.
Realms of Adventure is designed to be compatible with the Advanced Dungeons & Dragons game. Dungeons & Dragons was the 1st RPG with is origins dating back to 1974. As a result over the many years a great amount of useful supporting material has been published for it: adventure settings, world settings, monsters, spells, and magic items. So Realms of Adventure has been designed to take advantage of them and is fully compatible with those supplements.