Bacteria and viruses are often lumped together into a general category of "bad things that can cause infection". But these two types of infectious agents are very different, and the differences are important to your health.
* What Is a Cell? *
Bacteria are living single celled creatures. In contrast, viruses are not alive. So to clearly understand the difference between bacteria and viruses, it is important to first know what exactly distinguishes life from non-life.
All living things are made cells. These tiny units are the basic building blocks of life, and all living organisms are composed of at least one (unicellular) or more (multicellular) cells. A cell has all of the equipment that it needs to grow, turn food into energy (metabolize), divide and make more cells.
* The Two Cell Types *
There are only two main types of cells; eukarotic and prokaryotic. Eukaryotic cells (a.k.a. eukaryotes) are the cell type that we are generally more familiar with. Humans, along with other animals, plants, fungi, protozoans and a few other oddities are all eukaryotes. Eukaryotic cells are larger, more evolutionarily recent and more complex than prokaryotic cells. One of the most visible structures of eukaryotic cells, absent in prokaryotes, is a nucleus; genetic material (nucleic acid) enclosed by a membrane.
Bacteria are prokaryotic cells, which are typically smaller, simpler and are more evolutionarily ancient that are eukaryotes. These tiny life forms are nearly always single celled and are all classified as either bacteria (Eubacteria) or bacteria-like (Archaea) organisms.
* What Is a Bacterium? *
All bacteria are prokaryotic living cells, but all aren't bad guys. Our bodies are covered with many different species of bacteria, considered our normal flora. Normal flora help to crowd out pathogens ("bad guy'" microbes) and some even produce materials, such as vitamin K, that help us stay healthy.
Pathogenic bacteria are the ones that typically make us sick; some can even cause infections that kill. For example, Clostridium botulinum is the causative agent of botulism. This bacterium produces a potent toxin called botulin that can block nerve function. Botulinium toxin is one of the most poisonous naturally occurring substances in the world. Other examples of pathogenic bacteria that are frequently in the news include MRSA (a.k.a. methicillin resistant Staphylococcus aureus), Clostridium difficile (a.k.a. C. diff), and Vibrio cholera, the microbe that causes cholera outbreaks.
* What Is a Virus? *
Although viruses seem to display goal-directed behavior (invading cells and multiplying), they are not living things. To be classified as something alive, an entity must be made of cells; cells which metabolize nutrients, grow and divide. Viruses aren't made of cells and don't engage in any of these life processes.
Viruses are acellular particles made of at least a molecule of nucleic acid (genetic material) and a protein capsid (a container for the genetic material). Viruses are strictly parasites. They cannot reproduce without infecting a living cell and turning that host cell into a little virus factory. Viruses are never considered normal flora. They provide no benefit to the host cells that they occupy.
* Medications for Viral Versus Bacterial Infections *
Sometimes, if you've come down with an infection, and go to the doctor seeking pharmaceutical relief, your physician won't write a prescription for antibiotics. This is because antibiotics kill bacteria (living prokaryotic cells) but have no effect on viruses. Antibiotics are selectively toxic, they target either structures or metabolism specific to prokaryotic cells. This selective toxicity is why antibiotics don't harm human cells, but can still kill bacteria.
The common cold, influenza and a whole host of other illnesses are caused by viruses, and antibiotics have no effect on them. Most of the prevalent, non-life-threatening viral illnesses just need to run their course. For some of the more serious viral infectious diseases, there are vaccines to prevent infection (such as those that prevent polio, chickenpox or gential warts) or antivirual medications to lessen the impact of infection (such as Valtrex for herpes).
* Sources *
Bauman, R. (2004) "Microbiology".
Park Talaro, K (2008) "Foundations in Microbiology".