Movement of molecules from an area of high concentration to an area of low concentration is called diffusion. Picture a lot of people crowded into a room with the door open - the people will naturally spread out through the open door, to an area that is less crowded. Or if there is a crowd outside an empty room, some will naturally go inside if the door is open. That happens with cells, too. When there are too many molecules of one kind inside the cell, they will naturally flow out.
Osmosis is the diffusion of water across a semi-permeable membrane.
Lots of different kinds of molecules move in and out of the cell naturally by diffusion. During passive transport , molecules move naturally through a semi-permeable membrane from a high concentration area to a low concentration area. As in diffusion. There are molecules whose only job is to carry other molecules around. These are called carrier molecules .