This document describes in detail the basic concept of projectile motion.
When an object is thrown above the ground with an initial velocity u at an angle to the ground, it experiences projectile motion. In a vacuum, the only force acting on the object would be the downward gravitational force. The projectile motion is such that the horizontal velocity (V_x) stays constant while vertical velocity (V_y) shows that the object will decelerates until it reaches the maximum height, and then accelerates downwards back to the ground. Note that in 2-dimensional motion, the object has two initial velocities: one velocity in the x-direction and one velocity in the y-direction. In this discussion, we neglect air resistance.
The vivid examples given should be able to illustrate this concept conclusively.