This is a surface piercing at the nape of the neck. To my knowledge, they are always done horizontally; a vertical piercing would be under pressure every time the head was moved, and so would never heal. If done well and healed successfully, it just looks like a pair of metal balls about an inch apart, on the back of the neck. It is, for my money, one of the most attractive body piercings in existence.
The risks for this piercing are the same as for any surface piercing: high rates of rejection and migration, which then leave scars. These can be reduced by wearing the correct jewellery; a surface bar or flexible tygon, rather than a conventional barbell or captive bead ring. Good aftercare and not playing with/changing the jewellery will also help. Infection is, of course, a Bad Thing, but (as far as I know) no more of a risk for this piercing than any other. "You could hit a nerve and get paralysed" is still an old wives' tale.
Additional information from BMEzine.