Short for the Comics Guaranty Corporation, CGC is a grading service for comic books that emerged at the beginning of 2000. Similar in nature to grading services that already existed for coins and sports cards (and actually a sister company to graders for those collectibles), CGC takes comics for grading and returns them encapsulated in plastic holders with a grade assigned to each indicating the professional graders' assessment of its condition, on a scale from 0 to 10.
There has been a storm of controversy about CGC in all discussion forums frequented by comic fans and collectors. Some, who hold that comic books are intended for reading, intensely dislike the concept of sealing them in plastic. CGC defenders point out that it is in fact possible to open the holders, though this voids the grading -- if you want it resealed after opening and reading, you must resubmit it (getting a small discount for enclosing the label from the prior holder) and risk having the grade decrease the second time around.
The speculator and investor segments of the comic community can't help noticing the inflated prices CGC-graded comics often get, especially on auction sites like Ebay. If you have the good fortune to get a comic graded at 9.8, 9.9, or (the "Holy Grail" of grading) a perfect 10.0, you are likely to be able to sell it at many times the "Near Mint" guide price, even if it's a commonplace recent issue. This is hard to achieve, though; even brand new comics on the shelves of your local comic book store are probably mostly 9.6 at best, since at this lofty grading level even the slightest imperfections that arise during printing and distribution will mark it down.
Recently, CGC graded a giveaway comic from the 1940s (found in a big stack of papers that have been in a dentist office for decades) 10.0, making it the oldest 10.0 comic found to date (the previous record-holder was a Thor comic from 1968).