Seasonal (or common) flu

is a respiratory illness that can be transmitted person to person.
Most people have some immunity, and a vaccine is available.

Avian (or bird) flu

is caused by influenza viruses that occur naturally among wild birds.
The H5N1 variant is deadly to domestic fowl and can be transmitted from birds to humans. There is no human immunity and no vaccine is available.

Pandemic flu

is virulent human flu that causes a global outbreak, or pandemic, of serious illness. Because there is little natural immunity, the disease can spread easily from person to person. Currently, there is no pandemic flu.