What is a MMORPG?

M - ASSIVELY: The gaming world is large enough to sustain thousands of players simultaneously
M - ULTIPLAYER: Thousands of people play at the same time often co-operatively
O - NLINE: Requires a constant internet connection to play
R - OLE: Players often assume an online persona that is connected to their character
P - LAYING: Same as above
G - AME: It's a game!

Side note: "MMO" is also used as a shortened version of this acronym. It means precisely the same thing - think of it as an acronym of an acronym!

Clearly to be an MMORPG you have to meet the criteria listed above.

The concept of persistence in the online world in which the game is set is key. You, the player, would tend to pick up where you left off when you last logged out of the game servers. There is no reset button for your character or for the game. The maps do not change they are improved upon and set in a wider world for you to explore. The games World of Warcraft or Age of Conan would qualify as MMOs for instance, but the popular online multiplayer shooter Counterstrike would not.

Why is this?

When you create a character within an MMORPG you are expected to "build" that character over a period of time usually whilst interacting with the characters of other players. This can be done through a variety of means however the key is that it is usually done in real time and you would not normally expect to be able to play what is referred to as the "end game" two minutes after you have created your character.

As a player you are generally expected to learn the game as you progressively improve and upgrade your character (also called an avatar or 'toon'). You will in most likely scenarios not be killing a massive dragon or sieging a castle mere seconds after you have decided what hairstyle your toon will have. MMOs are meant to be played and developed over time with new content added for players to enjoy periodically.

In a game such as Counterstrike you would create your character and immediately proceed to playing on a map (not a world) with other players achieving the laid out objectives of the scenario - essentially beating the game within minutes or hours.

MMORPGs do not really have any such concept of beating the game. The game is normally ongoing ad infinitum for as long as the players wish to play (as long as it returns enough profit for the gaming company to continue to support it) and the game developers wish to continue adding content or increasing the cap at which your character can no longer progress or gain new abilities or skills etc.