Meg Masters was played by Rachel Miner in seasons 1-2. She was a young demon who was loyal to the Yellow-Eyed Demon.
Ruby was played by Genevieve Cortese in seasons 3-5 and then briefly by Katie Cassidy in season 6. She was a demon who initially presented herself as an ally to the Winchesters, but ultimately had her own agenda.
They are completely separate characters with distinct personalities and motivations.