The other posters are correct, but I'll try to clear it up a little bit more.
Coilover simply means the Spring (Coil) is OVER, the shock, or if you prefer the shock is inside the spring. Rather than some vehicles, or the rear of ours, where the shock is located independently of the spring.
To address the specific setup you're asking about, there is NO difference. Which has always left me wondering, as you are, why someone would pay so much for a toytec coil over setup when it is just a pre-assembled toytec spring and a bilstein 5100.
If you like this setup and are comfortable using spring compressors save yourself a LOT of $$$ and build them yourself.
If you want/need what some refer to as "true coil overs" (as mentioned: larger bodies, rebuildable, adjustable) they can be found for not too much more than the pre-assemble one from Toytec. I got my Icon setup for $100 more than the Toytec Ultimate Kit.