MMOs generally have quests that are the backbone of the game. Generally the point is to level up your character, usually by going out and doing a series of quests either alone or with a group of people. Some dungeons would feature harder enemy types that need to be defeated by a group, and generally in that group you have members who fill different roles.
One character might be the tank: a person who takes all the hits because they have higher defense than anyone else and sole purpose is to have all enemies attacking him, one character might be the healer, who uses defensive magic to protect all the other members from dying and is constantly healing the tank. Another type is damage, which focuses solely on doing as much damage as possible, but having to manage some kind of magic or other system constantly so they don't burn out on magic being utterly useless in later fights. Lastly, another character role might be considered support. Support generally consists of buffing allies with spells or abilities that improve their stats. Other support roles debuff enemies to make them weaker, easier to kill.
This explanation isn't geared toward XIV, but a general idea of MMO mechanics. The main point of any MMO really is grouping up with others to tackle harder quests in locations where you would be slaughtered by yourself. XIV will include other side things like getting a chocobo license to ride them around the world, a crafting system where you level up different crafting abilities like smithing, and a real-time economy where you sell and buy items where prices fluctuate depending on current market trends.
That's basically it in a nutshell, mankind. People get addicted to leveling up different classes since there are many different roles and ways to play, they get addicted to farming items to sell on a market, etc.
DAY 1!!!!! Hopefully this will be what I thought FF14 would be.