Slate is a softer shale like stone. The size and thickness can vary quite drastically. The 2 main differences when buying slate is gauged and ungauged slate.
Ungauged slate can vary from 1/4 inch thickness to 3/4 inch thickenss. This reason can make the installations very difficult as the owner should expect lippage as we cannot make ungauged slate level. So expect your floor to be up and down.
Gauged slate is a better product and more expensive. However gauged slate can still vary in thickness about 1/4 inch therefore it is easier to install of the two and your floor or walls should be better for levelness. You should still expect lippage however you shouldn't be able to notice it as much.
All slate jobs have to be sealed as this helps the look and lifetime of slate. Slate should not be installed in showers or outside as water can penatrate slate. If it is to be installed in showers you must install a waterproofing membrane first and must be sealed a few times.