The main and most important difference for man is that most bacteria are beneficial to humans. Our bodies contain thousands of bacterial species that outnumber by 10 to 1 our human cells and these help in food digestion, to synthesize vitamins and help promote our immune systems to fight disease. They have an important role in photosynthesis, nitrogen fixation and recycling nutrients, breaking dead animals, plants and fungal matter down into simple organic compounds.
A virus is 10 to 100 times smaller than a bacterium.
Bacteria are shaped by a rigid cell wall, containing peptidoglycan polymers, encasing a thin rubbery cell membrane that encloses the liquid cytoplasm. They also contain DNA, the genetic information that enables them to reproduce themselves. They can also contain plasmids, loose bits of DNA that can replicate separately from the main DNA strands and ribosomes that enable DNA to be copied. Some bacteria are also capable of self-motivation as they have external, thin thread like structures called flagella.
Viruses on the other hand are a simple structure of a protein coat and a core of genetic material, either DNA or RNA, not both.
Whereas bacteria can reproduce on non-living surfaces a virus must have an animal or plant to live on, as its host. A virus can only reproduce by invading and taking over a living cells capability for reproduction. This changes the cells genetic material from its normal characteristics into those of the virus.
Bacterial diseases can be treated with antibiotics, drugs produced from other bacteria or fungi that are able to fight harmful bacteria. These act by disrupting chemical processes, stopping the formation of cell walls and breaking cell membranes.
Antibiotics are not however effective against viral diseases as they live within the actual cells of the host, not as a separate organism. To prevent reproduction in this case would cause death, so the only method is for the host to adapt its own immune system to fight the invasion. This is why vaccines containing tiny quantities of specific, identified viral infections are used to successfully control diseases like polio, measles and mumps.
Bacteria also have the ability to remain dormant if subjected to a hostile environment, only re-emerging when conditions become suitable for reproduction again, such as the bacteria causing tetanus and anthrax.
So in conclusion, if you have a cold or other viral infection do not expect your doctor to prescribe antibiotics, as you now understand why they will be ineffective.