Fellowships are basically just a continuation of residency, but in a more specialized field. There's not a whole lot of difference between the two as far as work hours, pay, etc. A fellowship usually lasts anywhere between 1-3 years, depending on the specialty. There is a huge amount of variability as far as how many hours fellows work. In some specialties, such as cardiology or GI, fellows may pull longer hours than residents. In other specialties, like endocrinology or rheumatology, the hours are pretty much 8-5.
Fellows are paid more than residents, but not by much. At most hospitals, pay is based on what post-graduate year you are, so that interns (ie PGY-1), are paid the lowest salary. Pay raises are approximately $1000-2000 per PGY year. A first year fellow in a specialty in internal medicine (a 3 year residency), will be condisered a PGY-4, which is one year ahead of a senior resident in internal medicine, so will make about $2000 more than 3rd year residents at that hospital.
The process to get into a fellowship is similar to that getting into a residency. You typically apply during your last year of residency, but in some specialties, you apply during your second-to-last year. The app is the same thing as with residency -- you apply to whichever hospitals you are interested in, you do interviews, etc.
I can't be too much more specific than the above because fellowships are so variable depending on the specialty, but I hope this helps.
