Unless you've been disconnected from the global news for the past few days, you've heard of swine flu. Swine flu is basically the flu in pigs caused by a specific strain of influenza. The influenza viruses are a family of viruses that infect mammals (including people) and birds.
There are multiple strains of the influenza virus. Different strains have slight differences in their genetic code, which result in massive differences in the virus' features. Different strains are often specific to different species and some are better at hiding from their hosts immune system than others.
Influenza viruses are most dangerous when they are first able to infect a new host species. For example, our bodies have seen human influenza for years and our immune system has learned how to fight it. This is why most of us only have flu like symptoms for about a week after we are infected with human influenza. However, our bodies have never seen swine influenza because we aren't pigs (feminists may disagree with half of that statement). Thus, when swine flu is first able to infect humans, the virus is incredibly good at making us sick because our immune systems don't know how to fight it.
Swine flu has the potential to be a pandemic because it may be difficult for our immune systems to fight, which means it would be able to easily spread from person to person.
A very important point to remember is that all the infected patients in Canada have had mild symptoms, so this virus may not be as dangerous as we are fearing. However, it is responsible for over 150 deaths in Mexico, so health authorities are justified in their concerns about swine flu.