Animals are characterized as living organisms that feed on organic matter. From a dog to you. Animals come in all shapes and sizes, but all are made of the most basic unit of life, cells.
Animal cells are contained by a plasma membrane that holds all the organelles of the cell. In a lifetime, you will have had trillions upon trillions of cell that have made you up. In fact, every 7-10 years you are made of completely new cells (though the cells are clones of the old cells).