It is the cell membrane that keeps the insides in and the outside out. Though like your house it has doors and windows in it to let things in and out. A cell membrane is very flexible and the cell can change shape quite easily. However some cells have given up this flexibility for greater strength and protection in the form of a ‘cell wall’. A cell wall is not flexible so cells that have one have a constant shape.
What does a cell membrane let in? – http://wiki.answers.com/Q/What_does_a_cell_membrane_let_in