Maybe you notice that some cards have BGS on its title. Example #5 Michael Jordan 1993-94 Ultra Scoring Kings BGS 8.5 You can see the pictures on the left (click to enlarge). Sometimes if the seller sell the card they will describe more specific such: centering 9 Corners 8 Edges 8.5 Surface 9
This card are valued as NM-MT (Near Mint - Mint) you can see it below the value 8.5 and below the NM-MT there is a register number: 0004037062
Okay, some of you will ask, what's it all about?
BGS is Beckett Grading Services. It a services to grade a card for a specific value on the Centering, Corners, Edges and Surface plus Autograph. If you use beckett magazine as price guide, there are pages that explain about Condition Guide. Here are the explanation:
Centering is the measurement of the position of the cards image. A perfectly centered card is where the card's image is aligned exactly where the card company intended to be. Centering can be quickly judged by the naked eye. A border that is just slightly less than even on both sides of the card would measure 55/45. If one border is twice the size of the other, that equates to approximately 65/35. Because centering is an important element of the overall eye appeal of the card, centering can be a major factor in the cards overall grade.
Along with centering, the corners are one of the most important areas of a card when grading. A dinged corner drops a card's aesthetic value like a rock. To achieve a superior corner grade of GemMint or better, each corner must be blazingly sharp. Study your card's corners under magnification using a loupe. A 10x loupe is good for card grading. If the corners are all sharp to the naked eye, you are probably safe with a mint or higher corner. Once a speck of wear is visible, or a bump can be seen, the grade starts dropping fast. Always look very carefully around the whole corner not just the very tip. Often there may be an actual crease or bump across the corner, which may be difficult to detect at first, but once seen, becomes obvious. If any corner shows a bend or crease or other noticeable wear, the corner grade will likely drop to near mint or lower.
Unless heavily damaged, edges are often treated with general disregard by collectors. In reality, edges reveal more than just a grade- they are usually the key are in determining trimming or other alterations. Edges indicate how well the card was cut at the factory. While allowances are made for certain types of factory-cut edges such as O-Pee-Chee cards, the quality of the cut is taken into account to some extent. When it comes to after-market damage, the grading process is not as lenient. Chipping and notching that occurs after the card has been cut and packaged at the factory is weighed more heavily than factory problems in determining edge grades. Any large chip or notch in an edge is quite distracting and can lower the grade proportionately. To earn mint or better grades, the edges must be clean and sharp.
After each factors has been examined for example this card it has 10 for centering, 9.5 for corners 9.5 for edges and 9.5 for surfaces, you will have 38.5 in total (10+9.5+9.5+9.5). So it have 9.5 Gem Mint, it from 38.5/4 = 9.625. The 10 for autograph didn't included. It has separated examination. If you look at the first graded card on top, you will see it has 8.5. From the explanation below you will see it has Near Mint-Mint (NM-MT) because it bigger value than 8 the grade add + in the criteria. So it became NM-MT+
Gem Mint [9.5] & Pristine 
considered higher than Mint. Pristine is the absolutely perfect card, very very scarce.
No flaws, the perfect card. Four perfect corners, 55/45 or better centering, smooth edges, original color and gloss, no print spots, color or focus imperfections. A professionally graded mint card receives a large premium in price, often 200% or much more than Near Mint prices quoted in price guides. Many of the cards pictured in this book do not even in exist in this grade, and Mint examples would sell for lots of money.
Near Mint-Mint (Nrmt-Mt) [8 & 8.5]
Close, but not a Mint card. Centering must be 60/40 or better. There may be a minor flaw with the card such as a slight 'touch' (a light point of wear where the card was bumped or touched), the picture slightly out of focus, or a slight color imperfection. Nrmt-Mt cards also receive a premium in price over a Near Mint card. This premium is not as high as a Mint card. Again the cards from this book are rarely found in this high of grade. Cards from the 1990's are regularly found in this grade
Near Mint (Nrmt) [7&7.5]
Close, but not a Near Mint-Mint (Nrmt-Mt) card. Its centering can't be worse than 70/30. One of the following imperfections is allowable: very slight touch to two or three corners, slight wear to the edge, minor print spots or color imperfections. This is the standard grade which cards are measured buy in the hobby. The prices quoted in most books are for Near Mint cards.
Excellent-Mint (ExMt) [6&6.5]
Card may have visible surface wear or a printing defect which does not detract from its overall appearance. Corners may have slight fraying. The picture may be slightly out of focus. The card may show loss of original gloss or have a minor wax stain on the back. Centering must be 80/20 or better.
Excellent (Ex) [5&5.5]
Centering must be 80/20 or better, with four fuzzy corners. It may also have rough edges, minor discoloration, print spots or focus imperfections.
Very Good (Vg) [4&4.5]
This card shows wear, but still retains some attractiveness. Corners may be somewhat rounded. There may be edge wear, border discoloration, hairline creases.
Good (Gd) [3&3.5], Fair (Fr) [2&2.5], Poor (Pr) [1&1.5]
The bottom rung of cards. Good is the best and Poor is the worst. A Good card has significant wear, such as severe corner rounding, creasing, fading, staining or other problems. Altered or retouched cards will fall into this category). A Good card still retains some attractiveness. A Poor card has extreme problems, such as a significant part of it is missing, or ink or paint has marred it. The difference between a Fair and a Poor card, is that a fair card has some semblance of appeal while a Poor card is utterly repulsive.
And the last is, you can check all the Graded card online! Yes, just visit this site http://www.beckett.com/grading/lookup.asp and enter the register number. In there you will also found the Population Report of similar card.