Suppose an object is thrown parallel to the ground. The gravity acts downward (ie. perpendicular to the direction of motion of the object). The work done by gravity on that object will be given by :
$\text{Work}, W = F \cdot S = FS\times\cos \theta$
$\implies W=FS \cos 90$ ($\because$ direction of gravity $\perp$ direction velocity of the object)
$\implies W= FS \times 0$ ($\because \cos 90 = 0$)
$\implies W = 0$
From the calculation above, gravity will not do any work on the object. So why does it follows a parabolic path and eventually falls down.