There are 2 types of Coloboma,
anterior and
posterior.
Anterior coloboma is what I have, in which the iris is slightly misshapen, giving my pupil a ice cream cone/comet shape. Other forms include a keyhole shape. It's hard to notice if you have dark brown eyes. It doesn't usually impair vision, although it makes staring at the sun a lot worse.
A posterior coloboma is found in the back of the eye, where the retina and optic nerve are malformed or have suffered trauma, ie. Retinal detachment. Somtimes laser eye surgery can fix it, sometimes not. It's not visible without a slit-lamp retinoscope, meaning an Ophthalmologist can tell looking at the fundus of the eye. I also have this too, and it impairs vision, potentially leading to blindness if untreated.
Usually one of these will only occur in one eye. It's very rare to see it bilaterally. (on both sides)