The red globe on the left hand side of the screen is the life globe. It is a visual representation of the amount of life force a character has.
As your character takes damage 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 life force and will die.
To see a point total representation of your character's current and maximum life force levels, hold the mouse over the life globe.