Any webhead knows that Spider-Man first appeared in the pages of Amazing Fantasy #15. But that issue's iconic cover wasn't drawn by interior artist Steve Ditko; Jack Kirby provided the pencils while Ditko did the inks. What fewer people know is that Ditko did create a cover for Amazing Fantasy #15, and now it's finally seeing the light of day as the cover for Amazing Spider-Man #700.
Marvel is billing this as the last issue of the series, and they're relaunching Spidey with some major changes next year (rumor has it that he'll be the new bearer of Doctor Octopus' tentacles, which is horrible and will be undone after about 16 issues), and to commemorate they've dug this cover out of the dumpster and repurposed it. It's one of the variant covers, anyway - Marvel's back in the variant cover business, having about 40 covers for Uncanny Avengers #1.
In the incredibly unlikely case that this is the last ever Amazing Spider-Man (they'll just change the title over and pick up numbering, so if the new title, Superior Spider-Man, runs 16 issues, they'll pick up with Amazing #717), this is a fitting capstone.