Water cooling kits are designed to allow you to install a water cooling solution as easily as possible. They come in two different versions - either as All-in-One systems, or as a kit with all the parts needed to create a simple water cooling system.
All-in-One systems are easy to install and have everything built-in to the hardware, with a closed loop so you don't even need to add coolant. They're the easiest form of water cooling for a CPU.
Kits come with everything you need to build your own water cooling setup, including pump and reservoir, radiator and fans, the block, tubing and fittings. These kits are guaranteed to contain all the correct parts for your loop, so you don't accidentally purchase anything that's incompatible, and will generally perform better and quieter than an All-in-One system.