When the moon orbits the earth, it has velocity. inertia would make the moon continue in a straight line instead of orbiting at that velocity. the fact that it orbits at the same distance t tells you that

There is an attractive force call gravity  

Explanation:

The reason the moon's inertia doesn't make it continue in a straight line is due to the attractive force of gravity and the reason the moon and earth's gravity doesn't make them collide with each other is due to the moon's inertia. To understand this phenomenon imagine that you have a ball tied to a rope, you keep the rope with one hand and with the other you push the ball so it can move freely; the ball, like the moon, has velocity, but the ball can not move in a straight line because the rope doesn't allow it, in our hypothetical scenario the rope is like the gravity that traps the ball in an 'endless' circle.

Note: The rope and the ball are a hypothetical case, it helps us imagine the phenomena but the forces involved are different than in the case of the moon.