Gretchin, or grots, are a sister species of Orks, smaller, weaker, less cunning but more intelligent than their fellow greenskins. Gretchin are a slave race, forced to do menial tasks and things the orks "Can't be bovverd wiv".
Gretchin are very similar to goblins in size and shape, and in fact probably descended from this species of greenskin.
Although an ork wouldn't admit it, their whole society would fall apart without gretchin. Who else will go and fetch ammo, clean vehicles, cook, grow food and such? Orks certainly wouldn't. Orks do let the Gretchin in battles, though their reasons are not so the grots can have the same fun as the Orks by any means.
Orks allow Gretchin to fight alongside them in battle, or rather, in front of, to soak up casualties and provide a distraction. They can also run back to get more ammo. Orks do, however, have other uses for the grots on the battlefield. For example, the puny ones are great for mine clearance. Just run the grots over the field to make the mines safe!
Gretchin are also used to man the ork artilery. While this may seem strange, remember that an ork loves nothing more than to get at the enemy up close and personal, so firing at the enemy at long range is very un-orkish.
Gretchin also often find themselves being "volunteered" for duty helping out ork mekaniks and Painboys (ork field surgeons), as either "Grot Orderlies" or "Grot Oilers"
Grots are also used by orks as a way to get better footing through difficult terrain (step on them).
As you've probably guessed, orks value gretchin very little, but "dat's OK 'cos dere's plenty left where 'dey came from."