Third-party grading is a way of life in trading card hobby these days. While it doesn't make sense on a financial level to get every card graded, those looking for top value in their high condition and star cards need to have a third-party grade and slab the cards.
However, many collectors send those cards off to the likes of PSA and SGC, among others, and hope for the best, not truly knowing what goes into the final grade – and often disappointed with the result because of this.
While many outlets provide the general parameters for what constitutes an EX (Excellent), VG (Very Good) or NR-MT (Near Mint) and the other steps on the grading ladder, there aren't a lot of sources for what grading companies look for in the cards that are submitted.
Enter Dave & Adam's Card World. The company that does a booming business for unopened product and its inventory size, has produced essentially the definitive guide to grading sports cards, complete with tons of visuals and formatted in an easy-to-read manner.
CHECK IT OUT here, bookmark it and refer to it often. It's great.