This aricle will give you alot to think about regarding coin grading. Some of this is book knowlege but most of what I will share with you here comes from my over 40 years of experience collecting, buying, selling, and trading coins. There are pretty set standards for grading circulated coins. However, the grading of uncirculated "mint state" coins is quite an art. The standards vary from collector to collector, dealer to dealer, and grading service to grading service. Not only that, but the standards for grading mint state coins changes over time. Grading is both technical and at the same time, grading is subjective.
First off, there are no if ands and buts about it, if you are going to be a serious collector and commit more than just a few dollars to your collecting hobby, then you must learn to grade. In this hobby, which is also very big business, there will be many opportunities to either make some money or avoid losing alot of money if you take the time and effort to learn to grade. Unfortunately, even for the most studious, commited collector, this can take quite awhile. What one must do is first study the grading references to understand the technical requirements of grading various coins. Then one must, through trial and error, spend considerable time examining as many coins as possible and attempting to assign a grade to them. If you have a more experienced collector nearby, or a coin club meeting, or an online coin forum where you can post a picture of the coin and ask other's opinions of the grade, then you should be able to expedite acquiring your grading skills, but still expect it to take alot of time and effort. This will be an incredibly valuable exercise as you learn from more seasoned collectors why your coin is or isn't the grade you assigned to it.
Another great resource is to get into the habit of trying to view slabbed (Professionally graded by a third party) coins without looking at the grade first. This is difficult to do, but if you train yourself with a little discipline, you can do it. Play "guess the grade" with yourself and with others as much as you can with these slabbed coins. Over time, combined with your reading about grading, this technique will allow you to start to pick up on the subtle nuances of the different uncirculated grades. You will start to get a feel for what the difference is between say an MS64 and an MS65 or the difference between an AU58 and an MS63. You'll learn that sometimes, a nice AU58 coin can be more desirable than a Mint State coin. You'll begin to recognize the quality of the strike, luster, and surfaces, and differentiate between ticks in the key focal points versus other ticks. You'll start to be able to judge "eye appeal". You'll learn to recognize when the toning on a coin is lovely natural toning or is something that just doesn't look quite right. You will be able to pick up on coins that have been cleaned or messed with. For now, just for purposes of learning to grade, we'll assume that these Professionally graded coins in slabs from grading services such as PCGS, NGC, AMBAC, and ICG are dead-on accurate. I'd say that most of them are. But a little later, we'll discuss how this is actually a very dangerous assumption in the real collecting world and why you still must learn to grade on your own.
Grading companies grade coins with human graders. Human beings do make mistakes. The grading services PCGS, NGC, AMBAC and ICG all do a very good job overall in grading and authenticating coins. However, they do make grading mistakes fairly often. They also occasionally make attribution mistakes. And, believe it or not, horrors upon horrors, once in a while, a fake or counterfeit coin gets blessed as the real deal by these professional grading and authentication services. But again, considering the volume that they handle, they do a very nice job grading and authenticating your coins. I recommend using them for more expensive coins unless you become expert yourself in grading and authentication. Not only will it give you a professional opinion of your coin, but you will also have a coin that is more marketable when you decide to sell.
Just be aware that if a coin is certified as a certain grade, there is some chance that the coin is not really what it indicates on the holder. How do I know this? Because I have seen many many coins in these holders that were obviously overgraded. I have seen a PCGS coin in MS64 that had corrosion. I've seen NGC give an MS65 grade to a coin with counting wheel damage. I've seen ICG mis-attribute a coin. Sometimes, coins are even undergraded. I have submitted many coins that I have cracked out of a 3rd party holder because it was obvious to me that the grade on the holder was just plain stupid. I haven't always been right myself as the coin would later come back the same or even worse. But, in the vast majority of cases, I was right and I would reap the satisfaction and financial benefit of an upgraded coin. I have also sent coins into one grading service and they "bagged" it for artificial toning or cleaning. I felt strongly that this was not the case. So, I sent it in to another grading service and they agreed with me resulting in a getting my coins graded and encapsulated. No grading service is immune from mistakes. I have sent coins that NGC bagged (wouldn't grade or encapsulate because they think its got a problem) that PCGS graded with no problem. I've cracked out ANACS coins that they said were cleaned and had PCGS and NGC say otherwise. I've had NGC tell me a coin was artificially toned when I knew it wasn't because I pulled it right out of its original mint set holder. Later, I resubmitted it to them with a group of other toned coins pulled similarly from mint sets. This time, it came back graded with no problem. In a perfect grading world, these things would not happen. By now, I'm sure that you get the point. Grading is both a science and an art. Often, there is alot of money on the line when it comes to the proper grade on a coin. You had better learn to grade for yourself if you would like to have some nice money making opportunities or to just avoid plunking down way too much money on some overgraded coin you need for your collection.
Best of luck with your grading education. It is a challenging but very worthwhile skill set to aquire. Just remember, no matter how proficient you become, you will never totally master the science and art of grading. But, you will be light years ahead of all those who haven't taken the time and effort to learn how to grade on their own.
If you want to look up any of these grading services, just do a search below and add "coin" behind the name (eg. search for "PCGS coin" to find the PCGS web site).