The cell membrane is selectively permeable. That means some stuff can enter and some stuff can leave.
The phospholipid head is hydrophilic (water loving) and the phospholipid tail is hydrophobic (water hating).
Passive transport does not require energy and moves stuff from a high concentration to a low concentration, but active transport does require energy and it moves stuff from a low concentration to a high concentration.
The three types of passive transport is simple diffusion, facilitated diffusion, and osmosis.
Active transport includes exocytosis (big stuff leaving the cell), endocytosis (big stuff entering the cell), and various pumps.
When molecules on both sides of the membrane move in both directions, the system has reached equilibrium.
Simple diffusion involves the movement of molecules from a high concentration to a low concentration. It can be any substance.
Facilitated diffusion is the movement of molecules through a channel protein from high concentration to low concentration.
Osmosis is the movement of water from high concentration to low concentration.
A cell placed in a hypotonic solution will swell. A cell in a hypertonic solution will shrink. A cell in an isotonic solution will stay the same.