Wearing a normal shirt when breastfeeding means you have to lift your shirt to give your child access. That leaves you open to a lot of exposure. A nursing top is different from a regular top because it features a hidden opening at the front that allows you the freedom to breastfeed or pump discreetly. Brands such as Milk Nursingwear offer three types of openings: lift up, pull down, or move aside nursing access. The openings are so discreet that you can't even tell that the shirt is made for nursing.
You'll want to also look for soft, breathable fabric, since a lot of times nursing can raise your internal temperature and make you feel hot and sweaty. Milan Maternity, Boob, Japanese Weekend, and Maternal America are all brands that offer light fabrics for mommies needing to nurse in public. Best of all, all of the brands on this list make breastfeeding shirts and nursing tank tops that look just like normal shirts so you don't have to feel you're in "mom gear" all the time.
If you're a new mommy in fear of breastfeeding in public, a nursing or breastfeeding top can answer all your concerns. Take a look at the list and vote up the nursing top brand that makes you feel as comfortable as possible.