There are two primary types of nail polish removers out there, pure acetone and solutions of acetone, water and other stuff, in the polish world we refer to them as non-acetone (even though they do have some).
100% acetone, also known as CH3COCH3, is a colorless liquid with a VERY strong smell. Non-acetone removers can be all sorts of colors and some are even scented to someone mask the strong smell of the acetone present. I switch off between the two for various reasons....let's break it down.
Pros: By far the fastest way to remove polish, also strips oil from nail so base coat adheres more. It also costs about the same as non-acetone removers.
Cons: Well, it strips your nails of oils so it can be very drying. Also, it's not really supposed to be used by itself- my Beauty Secrets (from Sally's) bottle of acetone says "manicurist solvent." Being in general chemistry, acetone is kind of a big deal in the chemical world because it's such an important solvent. What is a solvent? According to wikipedia and my chem book, it's a liquid or gas that dissolves a solid, liquid, or gaseous solute (pretty much anything that can dissolve in the solvent, science is pretty repetitive), resulting in a solution. So technically, it's supposed to be blended with something else. Acetone can be used by itself but the reason why it's mostly used as a solvent and not by itself is because acetone is an irritant. Wikipedia has a nice list of the possible long-term health effects of inhaling acetone fumes or even exposing your eyes to acetone fumes. I can vouch though that when we're handling acetone in the chemistry labs, we absolutely must wear our goggles because it can be extremely irritating. I have to stress that if you're using pure acetone though, you really should be in a highly ventilated area!
Fun Facts: Acetone is an organic substance, and it can be found naturally in our bodies. Also, it's a main component of paint thinners. Which means it's a main component of nail polish thinner too! My Beauty Secrets thinner lists it as an ingredient in acetate form.
Recommendations: Beauty Secrets (from Sally's): $2.49 w/o card, $1.99 w/card for 225mL/8 oz.; there are also generic versions you can get at Big Lots.
Pros: Not as scary as pure acetone, it's also less damaging because of added conditioners such as Vitamin E. Since the acetone is diluted with water and other stuff, there aren't as many health risks unless you drink it. And if you do, please call the poison hotline because that's a little scary.
Cons: It's definitely not as fast as pure acetone, and in today's society we are not programmed for patience sometimes. It can be frustrating especially for taking off glittery polishes.
Fun Facts: Take advantage of FDA regulation and check the list of ingredients (I think Canada and the EU have this sort of regulation as well). If the first ingredient is acetone, then followed by water, chances are that the remover will work better than a remover with the first ingredient being water and then acetone. Why? The first ingredient is usually the biggest percentage of what is in the package. If the first ingredient is acetone, there's usually more acetone than water; the reverse if true if water is the first ingredient. So it's still a diluted version of acetone but there's still more acetone than water so it's not as weak.
Recommendations: Onyx Strawberry Scented Remover: 99 cents at Walmart for 118 mL/4 oz; Sally Hanson Moisturizing Remover, Sally Hanson Strengthening Remover.
You can see then why I switch off between the two? Pure acetone definitely helps when I'm swatching a lot of polishes but for day to day use, I prefer non-acetone removers even if it takes longer.