Facilitated diffusion is a process whereby a substance passes through a membrane with a aid of an intermediary or a facilitator. The facilitator is an integral membrane protein that spans the width of the membrane. The force that drives the molecule from one side of the membrane to the other is the force of diffusion.
In the figure below K+ ions are passing through a membrane using a potassium transport protein. The ions are moving down a concentration gradient so the process is a diffusion of K ions.
Transport proteins are very specific with respect to the kinds of molecules or ions that will be transported. For example a Na+ transport protein transports Ka+ ions much better than it will transport K+ ions.