Crystal magick is the practice of using the Magickal properties of crystals in the craft, whether that is in mojo bags, in healing pendants, etc.
Crystal magick can be used in conjunction with other forms of practice or alone if you wish, each crystal has slight varying properties, these in conjunction with colour and other properties can be used for representation during practice.
I am sorry I could not say it any better.
I use crystals with any ritual I do. I know they are considered more of a new age but crystals are what opened the door for me. Some basics, dark stones obsidian, jet, onyx all are very good at absorbing negative energy but need to be cleansed or recharged. Golden Tiger's Eye is very grounding. I use stones that represent each of the elements. Using them can get very complicated. I would say a few good basic stones to start with and learn with are obsidian, Amethyst, clear quartz, golden Tiger's Eye, and lapis luzli