Influenzavirus A, Group v virus, Orthomyxoviridae, an RNA virus. Specifically:

• Order: Mononegavirales
• Family: Orthomyxoviridae
• Genus/Genera: Influenzavirus A
• Species/Type: Influenza A virus

See the related questions for more details.
Orthomyxovididae = influenza viruses

Type B Influenza facts and comparison with Type A influenza viruses:

1. Type B Influenzavirus is an infectious virus in the the Genera of Orthomyxoviridae (flu viruses). It is an RNA virus.
2. Orthomyxoviridae in the Genera of Influenzavirus Type A have been the pathogens that have caused all of the various flu pandemics since we have known about viruses as infectious agents.
3. Type B flu viruses are typically much less easily spread among people than the Type A viruses.
4. Type A influenza virus strains mutate 2 - 3 times faster, making it more difficult to prevent infections of Type A with vaccination, as exact matches to use them are less available after only a few mutations which makes them less effective in vaccines.
5. With Type B viruses, the severity of the symptoms are typically not as bad as in Type A, and the duration of the illness can be shorter than the Type A infections.
6. There are many more opportunities to contract Type A flu than there are for getting Type B flu, because there are so many host animals that can get Type A flu...more cases = more spread = more cases = more spread...
7. The only known types of animals that can be infected with Type B influenzavirus are humans, seals, and ferrets. Type A has significantly more hosts besides humans, including birds, especially wild water fowl and poultry, swine/pigs, turkeys, dogs, cats, ferrets, and a few others.