Originally Posted by Hellmark
nook is from Barnes and Noble, so supports their proprietary DRM, as well as the Adobe DRM. On a stock nook Simple Touch, nook Color, or nook Tablet, there is a limit on how much you can "Sideload". Sideloading is when you transfer a book from your computer, rather than downloading directly from the built in store on the device. For the built in storage, you can only sideload 256mb on the Simple Touch (of the 2 gigs storage), and 1gig on the Color or Tablet (of the available 8 or 16 gigs). That is still a lot of space, but that limit can be changed if you root the tablet.
All Nook devices you mention have an micro SD-card slot to add memory for sideloaded content. For $5 you can already buy an 8 GB card.