The series will actually follow a young Hannibal, a Hannibal long-prior to the one we met in Silence as an enemy/associate to FBI Agent Will Graham (a character played by Edward Norton in the Silence prequel Red Dragon). The series will take the form of a one-hour drama and will be written by Bryan Fuller (Pushing Daisies, Heroes).
There are several ways this series can transpire; any of which will likely take some liberties with the character histories as they have been understood so far (but that's to be expected from most prequels). The show can be a sort of cat-and-mouse ordeal, with Graham chasing Hannibal, and Hannibal toying with Graham. Perhaps, in the vein of The Silence of the Lambs' plot, Hannibal and Graham can "work together" to solve crimes...from this, we could end up with something along the lines of a much darker Law & Order-style procedural (highly unlikely, thankfully), or something reminiscent of Dexter.
Think about it. Dexter Morgan (Michael C. Hall) is a killer who works to solve murders—it's his intimate understanding of the killer's mind, in a way that only a killer can understand it, that makes him so adept at his mission. That's exactly how you could describe the plot of Silence. Of course, this is no groundbreaking realization. The comparisons have been drawn many times before. But perhaps the Red Dragon and Hannibal Rising producers, in light of Dexter's popularity, are aiming to cash in on this with the original Manhunter.