At the cellular level, osmosis is the process where particles diffuse over a semi-permeable membrane from a lower concentration to a higher concentration. A less-concentrated solution is called hypotonic, while a more highly concentrated solution is called hypertonic. Isotonic refers to a solution that is balanced.
When blood cells are placed into a hypotonic saltwater solution, fluid flows from the solution into the blood cells, which causes the blood cells to swell and burst. If the saltwater solution is hypertonic, then fluid flows from the blood cells into the water solution, causing the blood cells to shrink. An isotonic saltwater solution allows fluid to flow evenly between the blood cells and water solution, and the blood cells maintain a normal size.
Osmosis is a form of passive transport, which is a type of diffusion. Diffusion does not require the cell to use any energy other than thermal energy. Passive transport is spontaneous and determined by the permeability of the cellular membrane. Water can easily pass through all types of cell membranes.
Some molecules can only be transported through facilitated diffusion. Transport proteins assist molecules that are too large to pass through a membrane in this way. These proteins open special channels and change the shape of the molecules so they can fit into those channels.