The defining factor of an MMO is an online environment with a lot of players on the same server, with MMORPGs adding in the option to play as a character through a detailed story that evolves as you play.
While an MMORPG will always be an MMO, it doesn’t have to be the same in reverse, with some MMOs not featuring the RPG element and instead combining other genres such as first person shooter or real-time strategy. The defining factor of an MMO is an online environment with a lot of players on the same server, with MMORPGs adding in the option to play as a character through a detailed story that evolves as you play.