I'd seen and heard of nursing pads before I had my first baby, but it never occurred to me that I'd be in any rush to buy them.  I had nothing against them from an outsiders perspective, but until you actually start breastfeeding, it's hard to imagine liquids gushing forth from your nipples. 
That's why I was surprised when, on about day three after giving birth ( I was still in the hospital with baby), I woke up with rocks (they turned out to be my breasts) attached to my chest and the odd smell of rotting milk... which turned out to be the breastmilk painting rivers down the front of my shirt.  
So reason number one for nursing pads-- the SMELL. At least in the first few weeks when the milk is most concentrated.  If you are wearing nursing pads and they start to smell, you can always replace them with fresh ones. In my case, I didn't have any with me and I don't think I had a change of clothes left.  In had never crossed my mind to purchase pads before I went to the hospital, so I contacted a friend of mine who'd just finished breastfeeding and asked her if she could send over a few day's supply, which she did.
And so began my nursing pad education. First of all, these things can be found under almost as many names as brands, fabrics, shapes and sizes. Nursing pads, breastfeeding pads, bra liners-- they're all basically the same thing. What sets them apart from each other at the end of the day is how well they work.
I started out with disposable ones because I wasn't sure if I would need them for long enough to justify the cost of the washable ones.  I found that the price difference between the expensive, brand-name ones and the cheapie generic ones was huge. I couldn't imagine why I would opt for the more expensive ones-- at first. And then one day I looked down to see my dear son sucking away with what appeared to be a mouthful of foam. It turns out that he pad I had been sleeping with had disintegrated and bits of it were clinging to my nipple. As you can imagine, it was very distressing for me to see bits of bleached paper fibre inside my newborn's mouth. I vowed not to use that brand again and instead switched to another cheapie brand-- with the same results.  Once I figured out that these things tend to fall apart, however, I was a little wiser about letting my son latch on before checking the area for debris.  Still, it was time to move on.  There are some things in life that are just not worth dealing with, and disintegrating nursing pads are one of them.
By this point, I figured out that I was a big leaker.  Between this fact and the unfortunate disintegration incidents, it seemed prudent of me to switch to the more expensive pads. In the brand-name price category, there are pads that wick, pads that are breathable, pads made of all sorts of materials and combinations of materials. There are pads that boast noiselessness-- no crinkle crinkle. Some pads have indentations for your nipples some advertise super absorbency. Personally, I found that any of these things worked for me. I just had to remember to keep enough on hand to be able to change them as often as I needed. 
What's important to note is that yeast, aka thrush when it's growing on your nipple and your baby's mouth, will thrive in a warm, wet environment. So you either have to be diligent about switching pads as necessary or else you need to get ones that wick moisture away from your skin.  
Buying reusable pads was another step up. Obviously, there are environmental benefits to this, and there is the added bonus of not having to worry about all of the chemicals from the manufacturing process being transferred to baby since you'll probably be washing them quite often.
Things to take into consideration when choosing nursing pads:
- Shape and Size: These two features will determine how visible the pad is underneath  your shirt. So I guess it depends on the kinds of clothes you like to wear and also the season. Bigger pads are not necessarily more absorbent, but you may find them to be more comfortable and the seams might be less prominent when they're not pushed right up against the front of your bra cup. With a bigger pad, you also have to worry less about it shifting inside your bra and ending up out of reach of the leaky nipple. The thickness of the pad might also make a difference to how much it shows through.  There is such a thing as a thin yet absorbent pad.
- Fabric: The absorbency of pads depends largely on what the pad is made of.  The fabric or material also will contribute to how hot the pad feels, if it keeps the moisture next to your skin or if it's wicked away, and it can also contribute to the smell or lack thereof.  When it comes to disposables, you're mostly looking at papers, plastics, and cotton, though there are some that contain absorbent gels such as those found in baby diapers. Washable ones range from simple cottons to flannels, hemps, and even wool pads. There are even adhesive silicone pads available out there that somehow prevent leaks from happening at all and which can be worn in the water with bathing suits or without a bra-- now, that's a novelty when you're breastfeeding.
Nursing pad reviews are available on line, and some sites that sell pads will also have readers comments underneath.  If you're looking for the economy and sustainability of reusable pads but haven't decided  which ones to invest in, you can always invest in a box of disposables until you're ready to take the plunge. Disposables are also good for keeping handy in the baby bag for trips and for those unexpected events.