The inside of a cell is watery and jelly-like. Cells are very small - you can't see them just using your eyes. You need to use a microscope, which makes them look many times bigger than they actually are. If a cell is cut in half, it will not survive. So a cell can be considered as the smallest part of an organism that can survive on its own.
Some organisms have only one cell, while more complicated organisms are made out of lots of cells.
All cells have a membrane, which separates them from the outside world. The membrane protects the cell, and allows the cell to be selective about what is allowed in.