Missile path to target

Why do certain projectiles (e.g ATGM's) not travel on a direct path to the target, but go up/down/left/right'?

Is it a target-tracking thing; an inbuilt feature to lessen the chance of intervention by another missile, or something totally different?

My understanding is that it is the tracking of the target and the correct flight path. The guidance chip in the rocket is constantly figuring out where the target is, due to movement or obstructions, that chip constantly redirects the flight path until and desired flight path is aquired. Which is why you can see a missle or rocket straighten out as it gets near the target. Basically just a bunch of math equations going through the chips head until it impacts.

Joystick controled the rocket motors fire out the sides and back from the missle, constant recorrection by they operator is why you see this up down side to side plus what Brend said about avoidence systems built into the when this is operating the operator has to make constant corrections to the target.

Thanks guys....

Also, certain missiles like the Hellfire can be preprogrammed for a terminal trajectory to the target in order to hit weak spots or avoid obstacles. The first time I saw one fired, I thought it was heading away from the target. My jaw dropped as it turned and destroyed the target.