What's the difference between an MMORPG and MMO? Let's start by explaining what MMORPG and MMO stand for. An MMORPG is a massive muli-player online role playing game. An MMO is a massive multi-player online.
Now this is where it gets kind-of complicated. All MMORPGs are MMOs but not all MMOs are MMORPGs. I guess I should explain what a role playing game is. A role playing game is a game where the player assumes the role of a character in a fictional setting. Which makes things even more complicated. Because, even though you always assume the role of a character in all MMOs, certain MMOs like Audition for example are not considered MMORPGs.
Audition is an MMO where you represent yourself in avatar form. You compete against other characters in dance-offs and aim to be the best dancer. Along the way, you can earn experience to level up and gain ranks. You also get currency to buy your avatar self more clothes and hairstyles. But, this wouldn't be considered an MMORPG because the game doesn't really take place in a fictional setting. It's based to look like a real life place.
Now, games such as Ragnarok Online by Gravity and World of Warcraft by Blizzard are great examples of MMORPGs. In these games, you create a character and fight monsters. You get to become certain jobs such as a swordsman and acolyte and you train to become stronger and get higher class ranks (for example an acolyte turns into a priest then the priest turns into a high priest). You can fight with other people in a party to work together and kill monsters or you can fight against each other in the player versus player arena. This would be considered an MMORPG because not only do you assume the role of a fictional character, you also are in a fantasy-based land.
All MMOs will have chat rooms or some sort of chatting system so that you can communicate with other people. An MMO isn't considered an MMO unless you can have some sort of way to communicate with other players in game. Certain games will cause some dispute because some gamers might not consider it to be an MMO. Also, most MMOs you have to download. Certain sites like Neopets is not considered an MMO. MMOs are usually one huge game versus Neopets which would be an online community.
There are more genres to MMOs then just RPGs though. Other genres include simulation (Sims Online), real time strategy (Starcraft or Warcraft), puzzle (Puzzle Pirates), and many more.
The layout of MMORGs also differentiate. Some MMORPGs have you start out right in the fantasy land it takes place in. Others, like Grand Chase, have you start out on a world map and makes you join a room with 4 players to do dungeon crawls together. Games like Grand Chase are very hard to tell if it's an MMORPG or MMO. Just ask yourself, does this game take place in a real life setting or does it take place in a fantasy land? If it takes place in a fantasy land, it's usually an MMORPG.