If you do it wrong - nothing. As others have responded though, oxygen is the difference, and that is why you need to "turn" your compost every couple of days. If you don't, it will start to get very stinky - if you're doing aerobic composting shouldn't really have a smell, maybe something slightly earthy but that is it.
I’ve been composing for years without turning and without the stinky issue. My compost is connected to the chicken coop, they always have full access to the compost, maybe that has something to do with it.
All year I add everything, yard and kitchen waste, without turning. I stop adding beginning of spring, towards end of spring I dig it out for the garden. Easiest method of composting I’ve come up with, no fuss, great soil.
Just wanted to put out there there’s an easier way to compost.
