Mint State (Unc) - Absolutely no trace of wear.
Mint state coins vary from MS-60 to MS-70. As I stated earlier, MS-70 is perfect. No blemishes, good strike, great color, and a lot of other really nit picky stuff. Trust me, any coin you have that isn't slabbed isn't MS-70. It's like winning the lottery, twice. An average shiny new penny from the supermarket is MS-63. A really pretty one with no easily visible marks is MS-65. If you have an average roll of new cents from the supermarket, 2-5 will be MS-60, 5-15 will be MS-61, 25-40 will be MS-63, 5-8 will be MS-64 and 1-2 will be MS-65. If you are exceptionally lucky, one will be MS-66. An MS-68 coin is one in 100,000!
Distinguishing the difference between these uncirculated grades is where most of the black magic in coin grading is. Even with years of experience, coin dealers will disagree about these grades. Even the professional grading services aren't 100% consistent within these grade levels. Some coins are very difficult case studies. European coin collectors think that this American system of grading uncirculated coins is just downright crazy.
Almost Uncirculated (AU) - Small trace of wear visible on the highest points
AU coins come in AU-50, AU-55 and AU-58. As a novice, you probably won't be able to tell the difference between AU and Unc coins. I know I have trouble a lot of the time. The most important thing in grading an AU coin is to know is where the high points are on a particular coin so that you can look for the minimal wear there. Practice by taking a new coin from the bank, rub it back and forth on your mouse pad vigorously a few times, and see if you can see the wear. Hold the coin nearly sideways in a bright light so that the light reflects at a low angle off the coin. Look for a difference in how the light reflects from most of the coin versus the very highest points. If it doesn't reflect off of the high points the same way as it does from the rest of the coin, then you probably have an AU coin. Note that most AU-58 coins look much better than most MS-60 coins. One of the weird things about coins is that an ugly uncirculated coin often sells for more than a beautiful coin with barely perceptible wear.
Extremely Fine (XF or EF) - Very light wear on only the highest points.
XF coins come in XF-40 and XF-45. With an XF coin, you can usually see the wear without messing around too much, but it is a very small amount of wear. There is often some of the mint luster left on the coin. Most of the devices on the coin are clearly defined. For each type of coin, there are different things to look for in determining if a coin meets this demanding grade.
Very Fine (VF) - Light to medium wear. All major features are sharp.
VF coins come in VF-20, VF-25, VF-30 and occasionally VF-35. The key word here is major. Minor features such as some of the finer hair detail, feathers, etc. will be worn. Take a roll of quarters from the bank. Most of the coins from 1976-1983 or so will likely grade VF. Personally, I specialize in VF coins because they show most of the detail of the coin and are a fraction of the cost of higher grade coins. While they show honest wear, they are still very attractive and detailed. Note that silver and copper coins wear faster than clad coins, so VF probably represents between 1-3 years of use.
Fine (F) - Moderate to heavy even wear. Entire design clear and bold.
Fine is labelled F-12 in the Sheldon scale. Your average 1965 quarter from circulation is Fine. A lot of the details are gone, but you can still see a good deal of the design.
Very Good (VG) - Well worn. Design clear, but flat and lacking details.
Very Good is VG-8 on the Sheldon scale. The entire design is weak, but a few details are visible. Full rims are nearly always a requirement for this grade. A full rim means that you can see a line around the edge of the coin where it was raised up.
Good (G) - Heavily worn. Design and legend visible but faint in spots.
On some coins, full rims are not required for this grade. You must be able to read the date and mint mark.
Almost Good (AG) - Outlined design. Parts of date and legend worn smooth.
This is a used up coin. You should be able to make out the date (possibly with some effort). Often, only parts of the last two digits will be visible.
Fair (Fair) - You can identify the coin as to its type.
There may be holes, it might be bent, or it might just have a LOT of honest wear. You may or may not see the date depending on the type and the nature of the wear. Fair coins are also sometimes called "filler" coins. That is because you can buy them very cheaply to fill the holes in your collection. Otherwise, you might never be able to afford the coin. Many people collect fair condition coins, especially the rarer dates and types. Dateless buffalo nickels, for example, are still worth about a dime. Some of the earlier type coins may be worth $50 or more in fair condition.
Basal State (Basal) - You can identify the lump of metal as being a coin.
Basil state coins have extraordinarily low value. A basil state large cent, for example, might sell for a nickel.
Occasionally, you will find a poorly struck coin that looks like it has wear, but is really uncirculated. To see a good example of this, look at the German East African "gun metal" coins. The rudimentary mint at Tabora during World War I using a gun melted down from the SMS Konigsberg resulted in some nearly uncirculated coins looking more like VG. While there is a premium for well struck coins, the primary determination of value is from wear. Some coins are nearly always weakly struck.
Coins with scratches, holes, very dark toning, pitting, corrosion, fake color, retooling, repairs and other problems are severely downgraded.
Sometimes you will see coins graded in in-between numbers such as F-15 or VG-10 or VF/XF. There is typically not very much of a premium for these in-between grades, so in many cases they can represent a good value.
Some early silver and gold coins have adjustment marks. These do not effect the grade, but can affect the value in a similar fashion.