The Nook Tablet is Barnes & Noble's successor to the Nook Color, and was superseded by the Nook HD. Use this tag only if your question is SPECIFIC to the Nook Tablet.
The Nook Tablet is a 7-inch device featuring the characteristic cut-off corner of the Nook series. It's part of Barnes & Noble's attempts to mate their book-selling business with the growing e-reader market. It runs 2.3-gingerbread, heavily customized by the manufacturer to lock down many settings; but it can be rooted (cf. rooting) and can run cyanogenmod.
See also nook-color and nook-hd, and find more information on the device's Wikipedia page.