What Exactly Is Minecraft?
It's a game about exploration and building, using the blocks you find and mine then build creative fun things!
There are single player and multi-player and online games. Most purchased Minecraft games are off-line and can be played in single player mode. Read the details of the game you are purchasing to know whether you can play with others online or on what platform (iOS, Android, PC, XboX, etc).
The main objective is to kill the Ender Dragon but you do not have to play it that way. Most kids enjoy just mining blocks and building houses, light-houses, statues, farms, castles, under-water building, etc.
It's an open-ended game leading to endless hours of fun!