It seems as though zombies use sensory organs that remain mostly internal - if they lose an ear, they can still hear out of the hole. If they lose an eye, they can't see out of it - also the eyes (in makeup) tend to look all glassy and white meaning they're likely mostly blind. They have, in the show, demonstrated that the walking dead use their sense of smell quite a lot in determining who/what to eat... I think it was episode two or three where they slathered themselves in guts to get by an army of walkers... only to be sniffed out when the rain came.

EDIT - Somewhat ninja'd

In response to the 'herd' reflection - it's been postulated that if one zombie starts to walk toward a half-heard sound or light or somesuch, others will begin to follow it, and as it passes others they will also start to follow, and so on and so on - until there is a herd... In the show, Andrea just named it this for the first time it would seem.