Strategic Warfare means warfare using WMD's.
Strategic Warfare refers in SP1 to any strategic missile, either with a bio chemical, conventional or nuclear warhead. In SP2, it refers to only nuclear missiles.
Strategic weapons include ICBMs (Intercontinental Ballistic Missiles) launched from your countries soil and SLBMs (Submarine Launched Ballistic Missiles), launched from SSBNs (Ballistic Missile Submarine). Both weapons have their advantages and disadvantages.
The ICBM is usualy very powerfull, has a great range and is cheap. On the downside, it has to be deployed on your soil and if your country is invaded, you loose control of them.
The SSBN cannot be targetted by nuclear missiles as fleets cannot be targeted (unlike in SP1). They possess several missiles, and can be redeployed for more, meaning virtually unlimited missiles. Yet, they are very expensive and easily destroyed by fleets. Also, the range of their missiles is smaller.