The graphics card has a 2D mode and a 3D mode.
Browsing the net, as in your example, would use 2D mode which requires considerably less graphics processing power. When Windows starts up it's already in the default 2D mode.
Games use 3D mode which may use all of the graphics card's graphics processing power. The graphics card will switch into 3D mode when the game is launched and revert back to 2D mode when you exit the game.