THE DIFFERENCE BETWEEN BACTERIA AND VIRUSES:
Perhaps the most striking difference is that viruses, unlike bacteria, are technically not living organisms. According to the definition of what "life" is (that is, the definition accepted by most scientists) and among other things, a species must be able to reproduce by itself in order to be considered alive.
Viruses can't. They need to infect a living cell (meaning that they get inside and then take over the cell) and use the cell's "machinery" to make copies themselves.
On the other hand, bacteria have all the tools necessary for reproduction. They don't even need a partner, since they can reproduce by simply splitting into two. This will form two bacteria with identical genomes.
In order to do this, bacteria need to have a complex set of mechanisms that allow it to grow, make a copy of everything that must go into the new cell (such as the cell's DNA, or genome) and then actually divide.
Viruses are much simpler, as we will see in the following section.
Bacteria, the most simple of the living organisms, are actually quite complex. They have a chromosome made of DNA where the "blueprints" for everything on the cell are stored. They have some proteins that act as the "bones" of the bacterium and others that act as enzymes (they help build and fix the cell, as well as process the nutrients it acquires). The bacterial cell is surrounded by a membrane made of phospholipids and they are also covered by a sugar-rich cell wall that helps protect the bacteria. All these parts act in unison to help the bacteria adapt and survive in the environment. This environment may be anywhere from the bottom of the sea to the top of a mountain, from your skin to your intestine. Bacteria are everywhere.
Viruses are much simpler. They are made of a protein "capsule" that helps protect the small viral DNA and insert it into the cells it will infect. This DNA, much like a computer virus, contains a 'code' that takes over the cell and forces it to do what the virus wants. And what the virus most often wants is copies of itself, many of them.
A pathogen is an organism that can cause a disease in another. For example, the flu virus is a pathogen, like the bacterium S.pneumoniae which can cause pneumonia.
While viruses are universally pathogenic* (they always cause a disease because they MUST infect a cell to make copies of itself) some bacteria are actually beneficial to their hosts.
Examples of beneficial bacteria are those found in most mammals' digestive tracts. They help the animals by breaking down foods that the animals could not digest by themselves.
*(For more advanced readers, it could be argued that lysogenic bacterial viruses may be beneficial to the host cell, but they ultimately reach the lyctic phase and kill the cell anyway)
Most bacterial infections can be treated. Different kinds of antibiotics have been developed in the last 60 years, most prominently penicillin. A new and worrisome happening have been drug-resistant bacteria. These are new strains (families) of bacteria that are not affected by antibiotics that used to work against them. Resistance is caused by mutations, changes on the bacterial genome that somehow affect how effective an antibiotic is. For example, an antibiotic that fights bacteria by interfering how the bacteria produces a specific protein will be useless if the bacteria starts producing it another way.
Viral infections are more difficult to treat. Since viruses are such simple organisms, they are more difficult to treat. Furthermore, they live inside the cells, so any drugs need to get inside the cells as well for them to be effective. Furthermore, viruses may mutate (and so acquire resistance to a drug) even faster than bacteria, so any specific drug may be completely ineffective within days.