Traditionally, comic books covers have has a thicker stock than the insides of the comics, and are printed separately. That used to be glossy-newsprint but these days is mostly glossy-thick and glossy-thin.
"Self cover" prints cover and insides on the same stock, and the cover is printed as part of the internal pages. Which also means the page count drops slightly – instead of 24 or 32 pages, with four pages of cover, now those four pages will be printed as part of the multiple-of-eight page count.
Well, now Marvel will be publishing all their $2.99 comics in the 32 page self-cover format.
As advertising page numbers in comics, Marvel comics especially, have dropped away to almost nothing, and Marvel have shrunk the page count of their comics, the dropped four pages won't be noticed. The internal page stock thickness is meant to rise slightly as the cover thickness drops, but the comic will feel thinner as a result. Also, it might not be up to reading as easily in a hot bath.
The format becomes policy at Marvel from the first week of April.
Beginning with 4/4/12 On-Sale titles, all $2.99 comics will be 32 page Self-Cover format.