The movement of molecules across a membrane without the input of energy is known as passive transport.
When energy (ATP) is needed, the movement is known as active transport. Active transport moves molecules against their concentration gradient, from an area of low concentration to an area of high concentration.
Sometimes, molecules cannot move through the cell membrane on their own. These molecules need special transport proteins to help them move across the membrane, a process known as facilitative diffusion. These special proteins are called channel proteins or carrier proteins.
Passive transport does not require energy input.
An example of passive transport is diffusion, the movement of molecules from an area of high concentration to an area of low concentration.
Carrier proteins and channel proteins are involved in facilitated diffusion.