Nintendo Switch is a hybrid portable console with a North American release date of March 2017. It is the successor to both the Nintendo 3DS and Wii U, as Nintendo will no longer separate their portable and console devices. It is not backward compatible with the Wii U or the Nintendo 3DS.
Nintendo NX is the secretive successor to Wii U. There are a lot of rumors, but there's very little known about the system. It's rumored to be a hybrid handheld and home console.