eReader = device whose primary function is to read books ie NookColor, Kindle, NookTouch, etc.
Tablet = device that is touch screen based computer running a mobile os ie: IPad, Asus Transformer, etc.
There are no clean divisions, heck you could call the Asus Transformer a netbook...but the Nook Color is most definitely an eReader unless you do something to make it otherwise (i.e. root it, dual boot to cm7, etc)
IMO at least.
