Osmosis is the movement of water across a partially permeable membrane from a high water potential, to a low water potential.
Water potential basically means the percentage of water there, so pure water would have the highest water potential, and would move into an area which, for example, had sugar dissolved into it.
You can relate this to cells in the body, and that's where hypertonic and hypotonic come from.
If a substance is hypertonic then it has a lower water potential then a cell in the body (ie- it has more things dissolved into it, like salt water) so water will come out of the cell (down a water potential gradient) making the cell shrivel (become crenated)
A hypotonic solution has a higher water potential (there's less dissolved material, like pure water). Because of this water will enter the cell though osmosis and it will burst (become haemolised)
So, if I placed a cell into sugar water it will crenate because it's a hypertonic solution and has a lower water potential, but if I put it into distilled water it would burst because it's hypotonic and has a lower
