The comics that can be downloaded are usually in one of three formats. The first, .pdf, is the ever popular Adobe Portable Document Format and uses a .pdf reader to view the file. Most companies use .pdf's to send out their review copies as they are a more secure format than the other two.
The other two formats are very similar in how they operate and are known as .cbr and .cbz files. They use RAR and ZIP compression formats respectively (hence the R and Z in .cbr and .cbz) and house images that certain programs use to show the comic book file. These two file formats are the preferred method for illegal or pirated comic books as they are usually just scanned .jpg image files that others can access and manipulate if they want.
The second way that digital comics are viewed is through some kind of comic reader software that usually uses Adobe Flash to show the image files. Most every online comic store uses this type of format to show the comics people buy. The main reason for this is that the Flash files can not be redistributed to others and cannot be stored on computers to read offline. This ensures that a user will not be able to purchase comics through an online store and then give them to friends or distribute them via various pirating sites.
Some see the digital comic book as the death of the traditional comic book. Others see this as just another way to make comic converts.