In addition to the features listed below all of the NUCs feature the following:
The lower end entry-level NUCs feature an Intel Celeron or Pentium CPU. They typically consume little power and are very quiet. Good for micro servers, digital signage, HTPC and use cases where high CPU power is not required.
The Mainstream NUCs have the traditional 4" x 4" NUC form factor. They're powered by Intel Core CPUs and are available in compact and tall cases. The tall case has a slot for 2.5" SATA drive. They make an excellent desktop replacement.
The gaming NUCs are equipped with a powerful CPU and a discrete GPU that provides good capabilities to run even latest games. Very good connectivity on these NUCs as well.