Steve Simon has posted an article on Petapixel about back button focus on Nikon SLR cameras. Whilst much of the information presented in it relates to the D500 specifically, he highlights that by decoupling autofocus from the shutter release the photographer can lock focus at a specific point or track moving subjects by keeping the button depressed (this works really well with 3D or AI Servo Modes).
Canon has also posted an article detailing how to set up their cameras for back button focus.
While most top end cameras now have dedicated AF ON buttons, many cameras can have a button reassigned to provide this function. In addition, most housing manufacturers provide access to the control.
Mike Hagen of Nikonians and Visual Adventures has also released a video about back button focus:
As has Steve Perry of Backcountry Gallery:
Canon have also posted an article detailing how to set up their cameras for back button focus.