Most HUDs are a screen mounted in front of the windscreen.
MHD is a helmet that displays via a camera mounted external and normally moves the camera with your head and can be used for targeting.
A HUD normally shows avionic details, like RADAR, heading and the like, and weapon bus details of lock pickel .
Interesting note: When working in the UK our HUD for the F-22 required the pilot to wear a kevlar dickie to prevent the hud from killing the pilot by the glass HUD from a large bird strike when the windscreen flexes it can hit the HUD.
Get quick answers from Air Force Armament Museum staff and past visitors.