[Rogue Trader] Big Guys performing HMG Suppression Fire from the hip . . . NOT.

[Apache]

Overwatch is waiting for a target to show up, and then shooting at the target.

Suppressive fire is being aware that the enemy is Over There, and shooting at him (even though you know full well he is behind solid cover) to keep him pinned down (ie, suppressed). This doesn't necessarily mean burst or full-auto fire; semi-automatic weapons can (usually) put out enough fire to convince a target to keep his head down while your manuever element sets up their kill shot (or whatever it is they are doing).

A classic example of suppressive fire is shooting at a pillbox; the odds are pretty slim that you will hit anybody through the vision/firing slots, but the sound of rounds impacting near the slit will cause the gunner to flinch, and maybe even duck. And there is always the chance that you might get lucky........

And then there is my favorite, recon by fire...but that's an entirely different topic.