Tactical nuclear missiles, commonly designated “nukes,” are one of the most devastating attacks in the terran arsenal. Nukes can eliminate armies in one shot, wipe out entire worker lines, and cause heavy damage to structures.
To call down the thunder you'll need ghosts, which are trained at the barracks and require a ghost academy, and a factory (to enable your ghost academy to build nukes). It is also useful to research personal cloaking, otherwise your ghosts will be sitting ducks while painting the target. Each ghost academy can house one nuke. Some terran players build multiple academies in order to have a healthy supply of nuclear missiles and have the capacity to simultaneously nuke different objectives.
To prevent enemy nuclear strikes you'll need to kill the enemy ghost, and you'll need detection to do so (missile turrets in key base locations and scanner sweeps). Once that's taken care of, negate your opponent's nuclear arsenal by destroying his ghost academies.