Marnie, shedding goes like this...

First you may notice the snake's colors looking a bit dull for a week or two, then the eyes go cloudy(and sometimes the belly) for a few days, then they clear up. The snake should shed within a couple of days after the eyes go clear. The skin should come off in one piece like a sock pulled inside out.

A retained shed is when that last part doesn't happen for some reason. If the snake fails to shed, the old skin gets fused to the new and it can be fatal.

Reptile Skin Shedding