Symport - Two different molecules moving in only one direction. Antiport - Two molecules moving in opposite directions. The net directional movement is away from the center of concentration. Diffusion is the net movement down the concentration gradient. In a solution of many different substances each substance diffuses down its own concentration gradient independent of the others.
Diffusion is a passive type of transport, it requires no additional energy to make it work.
Osmosis is diffusion of water across a biological membrane. Water is usually the solvent, hence the term aqueous solution. If a cell from a salt water fish is placed in a beaker of fresh water, the cell is said to be hyperosmotic to the water. If a cell from a fresh water fish is placed into a beaker of salt water, the cell is said to be hypoosmotic to the water. Passive form of transport caused by the help of carrier proteins specific for the molecules they transport.