With most digital copies, you can only download it for a limited time, like a year or so after the initial release. Once you do, however, it should be good indefinitely. Personally, I find the DVDs and digital copies that they include with most new releases nowadays to be utterly useless, but to each his own.
The digital copy is compatible with iTunes or Windows Media. You will insert the Digital Copy disc into your PC/Mac, and then use your unique code (which you can find on a paper insert inside the blu-ray case) to activate your digital copy.
As Jon stated above, there is usually a limited amount of time in which you can activate your digital copy. But once you redeem it, you have it forever.