The blue globe on the right hand side of the screen is the mana globe. It is a visual representation of the amount of mana a character has.
As your character casts spells or uses certain skills and weapons, the level in the globe will drop or raise. When the level in the globe is at the bottom, your character is out of Mana.
To see a point total representation of your character's current and maximum mana levels, hold the mouse over the mana globe.