Put lets you put one object into another. Only some objects can have things stored inside them. Finally, there are characters. These characters are who you interact with in your adventures. Players come in two varieties.
Players are people who play the game, while 'immortals' are players who run the game, and are staff.
Immortals will help with questions about how to do things, but are sometimes prohibited from helping you do things. Players can help you with both, but sometimes are busy themselves. Mob's can exist for a number of reasons. The biggest reason mobs exist is to give players something to fight. You advance in the game by gaining experience, which is primarily done through killing mobs.