Without Osmosis, life as we know it would not exist. But what exactly is osmosis? The word has made it's way into the common vernacular, and in that context it means "to automatically and effortlessly absorb through contact". That's a good approximation, but the actually physical process is a bit more complicated.
Solvents, Solutes, and Solutions
Osmosis involves the interaction of solvents and solutes. A solvent is any medium that can dissolve and hold a solute. This mixture is the solution. It is also a homogenous mixture, meaning that the particles of solute distribute themselves more or less evenly through the solution. A good example of a homogenous mixture is salt water. Salt crystals, made up of sodium and chlorine, will dissolve in water so that the sodium and chlorine atoms will separate, become the solute particles.
In comparison, a heterogeneous mixture is a mixture where particles do not self-distribute evenly. A good example of a heterogeneous mixture is italian dressing, where the oil and water constituents tend to unmix after being shaken and the spices collect at the bottom of the bottle.
The spontaneous and even distribution of solute particles in a solution is an important concept for understanding osmosis. This is associated with the concept of entropy and the second law of thermodynamics. The particles can move to a preferred distribution and the total system loses a little bit of energy in the process.
Membranes
A semipermeable membrane is a thin layer that is impenetrable by particles of solute but not solvent. Two examples are the cell walls of organisms and on the macroscopic scale, a coffee filter.
In Osmosis, a solution comes into contacted with a semipermeable membrane and the solvent will be extracted, leaving a more saturated solution behind. Osmosis is a process that unmixes a solution.
In a situation where the solution inside the membrane is more saturated than the medium outside, the outside medium is considered hypotonic.
In a situation where the solution inside the membrane is less saturated that the medium outside, the outside medium is considered hypertonic.
In a situation where both the solution and the medium are equally saturated, the medium is considered isotonic.
Isotonic means that there is an equilibrium between the inside and the outside of a system. Hypertonic and hypotonic are states of disequilibrium, and the liquids work through entropy to reach a state of equilibrium.
The Three Little Cranberries
In order to remember the different states important in osmosis, consider the following "fairytale":
The first little cranberry, named Hypertonic, stayed out in the sun too long. He lost all the water from inside his little red body and became shrunken and wrinkled. His body became plasmolyzed.
The second little cranberry, named Hypotonic, fell asleep soaking in the hot-tub. When he woke up his body had absorbed a lot of water and he was fat and his skin was very tight, ready to burst. His body became turgid.
The third little cranberry, Isotonic, was smart. He stayed inside all day and studied his physics textbook. His body remained flaccid.
This isn't an exact match to what happens in osmosis, but it will give you a visual hook to remember the results on a cell immersed in each type of medium.
