The game has a very simple structure; go around and receive "tasks/quests" from various persons (controlled by other players) and solve these tasks/quests by killing animals and monsters, thus getting rewarded by "items".
The ultimate goal of the game is to gather more and more powerful items and make you character more powerful. To gather all the best items probably takes years(!).
Diablo and Diablo II had both the same structure as World of Warcraft. I liked them both but not World of Warcraft.
There are many reasons why I don't like World of Warcraft:
1)World of Warcraft costs money per month
2)It is very evident that the game is constructed to be as slow as possible so that people never reach the ultimate end. For example every enemy can take 10, or 100, or even 1000 hits to kill (!). Sometimes you have to gather insane amounts of items to solve quests.
3)The action is very slow, and your character can't die (just turns into a ghost for a short time), so there is never any danger to speak of.
10 years into the future everybody will probably hate how much time they wasted playing this garbage.