It is passive process and happens without any expenditure of the energy.
It involves movement of molecules from a region of higher concentration to the lower concentration until concentrations become equal on either side of the membrane.
There are 3 different types of solutions:
Isotonic solution: An isotonic solution is the one which has the same concentration of solutes both inside and outside cell.
Hypertonic solution: A hypertonic solution is one which has a higher solute concentration outside the cell than inside.
Hypotonic solution: A hypotonic solution is one which has a higher solute concentration inside the cell than outside.
Endosmosis is when the cell is kept in hypotonic solution, water flows inside causing it to swell up.