My grandson wants a LEGO motor. What should I get? My grandson wants a LEGO Power Motor to use with his gigantic box of Lego pieces.  He is 11 years old and seems to know his way around the world of LEGO bricks.  I, though, haven't a clue what he is asking for. I see things such as "EV3, NXT, RCX, and NXT-G and I'm confused. Can anyone help me?  What is it I'm looking for?
You say he is looking for a "Power Motor", which sounds like he is referring to the LEGO Power Functions line. The bare minimum you need to get a spinning motor would be one of the battery boxes and one of the motors.

Battery Boxes:


AA Battery Box
AAA Battery Box
Rechargeable Battery Box (requires Transformer 10V DC)


Motors:


M-Motor
L-Motor
XL-Motor


(They also make a servo-motor and a train motor, but these are made for special purposes.)



The terms you listed however refer to the different generation of Mindstorms robotics.


EV3: Current generation
NXT: Previous generation
RCX: First generation (rather old and difficult to use with modern computers)
NXT-G: The GUI used to program the NXT robots.


Mindstorms do include motors, among other things, but are mainly geared towards use with robotics.