The MD's residency training is regarded as postgradaute training, equivalent to postdoc training for PhD graduate.

The MD's fellowship is usually/mostly for subspeciality training, unless it is stated as "research fellowship". there are 1-3 years of trainings depending on the subspecialty, most of them very much focus on clinical skills, fellows can opt their researches as clinical oriented or bench oriented. As I know, few likes bench work. In our fellowship program, there are totally 6 month dedicated research time out of the 3 year training, and publishing abstarcts is a must, formal pubilication is not a must. Our specialty is top 5 in US this year.

Just a little info of US MD/PhD. Usually it is of 7 years (3 years in research for PhD training, 4 year for MD traing).

Also, just so you know that, in US, there are several medical schools that run MD programs that are designed to pick up students directly from high school for 6 year training (2 in biology, 4 in medical training).