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Almost all bacteria are exceedingly small. There are many microscopic plants, animals and fungi as well... not everything small is a bacterium... but generally speaking a typical big bacterium is smaller than a typical microscopic plant, animal or fungus. (The viruses are the last stop on the scale... they are tiny even compared to bacteria.)
From Wikipedia we learn that a pea-sized ball of typical soil would contain many more bacteria than there are people in England.
While you'll never see a single bacterium with your naked eye, you will sometimes see small "piles" of bacteria, typically on a bit of rotting food. They are best avoided.... it would be unwise to try to grow some.
Under very good microscopes, an individual bacterium may look like a short rod, or a small sphere. There are few of other shapes to.
Rather than identifying bacteria by what they look like, microbiologists identify them by the conditions in which they will grow, or by whether or not they are stained by certain dyes.
Like fungi, we tend to remember only the bad things about bacteria. They do, for instance, cause many terrible diseases... food "poisoning", tetanus, leprosy, and many more.
On the other hand, again like the fungi, they are important in the processes of decomposition which clear away things that we need to have cleared away. They are also important to science in studying life processes. They are being used more and more in industry. They are an "organic" way of controlling some pests which would otherwise cause enormous damage to agriculture.
I advise you, for now, to lump a bunch of things into a "kingdom" and call them all "bacteria". If you go further with your study of biology, this is one of the "kingdoms" I am recommending to you which later you will learn that modern DNA studies have shown not to be as closely related as their more obvious characteristics would suggest.
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