What an excellent and far-reaching question! Your question sounds like you are really thinking hard and wondering about the basic differences between various forms of life on Earth. My answer is kind of long, but this is a very important question, so we should make sure you understand some background material first. Then I will answer to your question!

Who started putting organisms into categories ?
Quite a long time ago, scientists started trying to categorize different types of organisms based on how they look (phenotypes), their anatomy (morphology), and sometimes their behaviors... One man who started this biological categorizing method was named Carolus Linnaeus (lived in the 1700's).

Carolus Linnaeus and others discovered that there were 5 overall categories that these groups of organisms could go into (kingdoms):
Monera
Protista
Fungi
Plantae
Animalia.

Later scientists found that they could subdivide the kingdom category even further into
Phylum
Class
Order
Family
Genus
Species.

(Side note: In 7th grade, my biology teacher taught me to remember all these words and their order by the sentence "King Philip Calls Out For Grape Soda", which is a pneumonic - a phrase that reminds you of the first letters of something else you need to remember! Some people learned "King Phillip Came Over For Great Spaghetti" or "Kings Play Chess on Funny Green Squares" HA!)

What do we know today about the biological category method? Today's scientists have added one more step above the kingdom level in the category method (Domain), because they have discovered even more organisms that don't fit into any preexisting kingdom (cool, eh?)

Domains:

• Eubacteria (One-celled organisms called "true" bacteria, including the more advanced bacteria; for example: Streptococcus, the "Strep Throat" bacteria)
• Archaea (One-celled organisms called "ancient" bacteria, including extreme-environments bacteria...hey, what are extreme environments on Earth? Can you name one?)
• Eukaryotes (One-celled organisms (but not bacteria) called Protists; plus the other multi-celled organisms in kingdoms: Plants, Fungi, and Animals)
• Viruses (Not even one-celled! Freaky, half-alive, half-not alive (by our current "rules" book) subcellular parts that are very good at inserting themselves into the cells of other living organisms and using those cells to help them reproduce very rapidly (i.e., Rhinovirus which causes the common cold in Homo sapiens). Scientists are still learning more about these interesting organisms everyday...

This was a long introduction. There used to be 5 kingdoms (which I learned in school), but recently scientists have decided that the old kingdom Monera (included all bacteria) needed to be removed from the kingdom level, split up, and reclassified into two domains .

Scientists decided this from the new organisms they discovered and from genetic tests that revealed how living organisms are truly related (it's all in our DNA...). Based on these new phylogenetic classifications, some of the older hierarchical category levels for certain organisms have been totally switched around (for example: modern day birds are very closely related to some extinct dinosaurs...weird...we didn't know that before because they don't look closely related!)

Here are some ways the kingdoms, and the domains of eubacteria and archae are different from each other, and (gives you hints of what they can eat):

• Plants can make their own food (photosynthesizers, autotrophs) using carbon dioxide nutrients and sunlight, while Animals generally have to get their food by consuming other organisms (heterotrophs), and Fungi get their energy by eating dead or decaying material (saprophytes).
• Eubacteria, Archae bacteria, and Protists both have members that are autotrophic or heterotrophic, or both (how do you think this is possible?)
• Fungi, Animals, and Plants all need to live in aerobic (oxygen-containing) environments, and, but some Eubacteria and many Archae bacteria live in places where there is very little or no oxygen (where would this occur on Earth?). Some Protists can also live in anoxic (anaerobic, or no-oxygen) environments, but some need oxygen to live.
• Bacteria have no cellular nucleus like the Eukaryotic kingdoms do (organisms with cellular nucleuses), and their cellular parts are much simpler than Eukaryotes
• Eubacteria, Archae bacteria, and Protists are single-celled. Animals and Plants are multi-celled.
• Fungi are strange because some are multi-celled, but some are technically unicellular (single-celled).
  
  
  
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