If you have crafted the bow with some arrows, then you are ready to shot mobs. The bow shots wither skulls, the hitted mob or player will get the potion effect wither and there is also a explosion that can mine some blocks.
Difficulty easy: only explostion, wither effect, only enchanted potions for 15 seconds.
Difficulty normal: explosion + wither effect to hitted entity for 5 seconds.
Difficulty hard: explosion + wither effect to the hitted entity for 20 seconds.