Polar Bear Click on image for full size (22K jpeg) Image courtesy of Corel Photography

First, life may be "multicellular". This means they are made of many cells, like a dog or cat. Or they may be made of only one cell, like bacteria. Second, most forms of life get their food by eating other organisms. Other life forms don't have to eat because they can use the Sun's energy to make their own food. All life forms seem to need oxygen to survive, but that is not always the case.

Here's a brief summary of life on Earth.

The Animal Kingdom mammals elephants, rodents, etc arthropods insects, crabs reptiles is multi-cellular and complex	The Monera Kingdom bacteria archaea is procaryotic; unicellular, no membrane-bound organelle nor nuclei.
The Plant Kingdom trees, flowers, etc varies from multi-cellular to eucaryotic; unicellular, with membrane-bound organelle and nuclei	The Protista Kingdom Amoeba, etc. Protists can be unicellular, multicellular or colonial. Some move around and act like animals, others perform photosynthesis like plants
The Fungi Kingdom athletes foot, etc resembles animals; cannot make its own food through photosynthesis; consumes organic matter.