Felloship is basically like a continuation of residency, especially in terms of salary. Basically, fellows receive slightly more money than residents. Usually the pay scale is set based on your post grad year and increases with each year. So, at most most places, a 2nd year cardiology fellow (3 years residency + 2 years fellowship) earns the same as a 5th year radiology or surgery resident.
In terms of responsibility, fellows are somewhere in between a resident and an attending. Depending on the hospital, some fellows act almost like junior staff physicians. Their notes don't need to be signed off by an attending in most situations.