Virology
H5N1 is an influenza A virus. Other examples of influenza A viruses are H3N2 and H1N1, both of which are considered "normal", seasonal flu.
Influenza viruses have RNA as their genetic material. There are 8 genomic segments which code for 11 proteins.
From Wikipedia
Influenza hijacks the cellular machinery to produce copies of itself. Here is how the different genes and proteins depicted above accomplish this:
The hemagglutinin protein (HA) binds to receptors on cells. Once the virus enters the cells, the different parts of the influenza virion separate from the matrix proteins (M1 and M2) which are important in giving the virus its structure. Proteins from three genes - polymerase A (PA), polymerase B1 (PB1) and polymerase B2 (PB2) - are responsible for making copies of the virus genome and making viral proteins. The nucleoprotein (NP) gene codes for a protein that binds to viral RNA and helps move it from the nucleus of the infected cell to the cytoplasm. Once a large numbers of copies of the complete virus have been made, the new viruses leave the host cell using the protein made by the neuraminidase gene (NA). This cell dies but many new viruses now infect new cells. This process is repeated as more and more cells are taken over by the virus and then die. The function of the non-structural proteins (NS1 and NS2) are not completely understood but are thought to play a role in how severe the symptoms are. One way they may do this is to suppress interferons which are used by the immune system to fight viral infection.
Influenza viruses, including H5N1 evolve over time. See Random Mutation, Reassortment, and Recombination for more information on how this occurs.