You must be referring to the one year traditional internship that DOs do. I believe it used to be required for all DOs. However, now it is only required if you want to practice in one of the following 5 states: Philadelphia, Florida....(does anyone else know the other 3?). That is, you can't get board certification in those states if you don't do a DO internship year. However, you can still practice in any other states and get board certified there.
This internship is 12 months and you learn a little bit of everything during the year. This counts as your first year of residency so if you go into family practice, this would be your Post graduate year one. Then you would do 2 more years and be out of residency. Basically it's the same amount of years an MD would do in an FP residency with the difference being that DOs do different stuff the first year.
However, if you're from a state other than the required 5 like CA then you don't have to do one. If I go the DO route, I plan to go straight into an allopathic residency without doing a DO traditional rotating year. I will still be able to get board certified and it won't affect me unless I go to one of the 5 states like Florida. Then I won't be able to practice medicine there.
I'm not sure if I confused you more but that's basically how it is. Maybe someone else knows the other 3 states that require it as well.