I know there is alot of conjecture to whether there are 5 or 6 families, because in GF I there is mention from Tom Hagen that if you gun down a police captain you will have the 5 Familes against the Corleones, this is a pretty big indicator that there were 6. IF there were 6 then wouldnt' they have been:

1. Corleone
2. Barzini
3. Tattaglia
4. Cunio
5. Stracci

6.Bocchicchio - Mentioned in the book when they helped give Michael an airtight way over from Sicily.

Also Michael said after he found out about Tessio "It's a smart move -- Tessio was always smarter. But I'm gonna wait -- after the baptism. I've decided to be Godfather to Connie's baby. And then I'll meet with Barzini -- and Tattaglia -- all the heads of the Five Families"

Also at that time in New York there were 5 families in real life.

During the commision meeting, from the book, there are only 5 families mentioned:
Stracci
Cuneo
Tattaglia
Barzini
Corleone

During the film, in the scene with the commision, an additional Corleone family is mentioned from the Bronx and Brooklyn.

So are these the 5, aside from the Corleone family of Vito Corleone NY Families?:
Stracci
Cuneo
Tattaglia
Barzini
Corleone (Carmine's)

This is one of the most frequently posted threads on these boards. You will find a recent discussion here:
http://www.thegodfathertrilogy.net/ubb/ultimatebb.php?ubb=get_topic;f=1;t=004457#000001