Here's what a cartridge looks like compared to a strut/shock.
The cartridge is something the automakers and not KYB decided to use, it basically makes it cheaper for YOU the owner to replace them rather than replacing a whole strut assembly; Toyota decided to use cartridges on some of their sportier models (MR2, Celica, Corolla GTS, etc..)
The way the cartridge works is simple, take the old one out of the strut assembly and drop the new one in, you may have to use a bit of force as rust and gunk will probably make it difficult to remove it.