Theresa,
Categorical refers to programs where you are committed for three years. These typicaly prepare you for the practice of general internal medicine, though depending on the particular residency program some or most of the residents may persue further training with a fellowship (ie cardiology, gastroenterology, hematology/oncology, etc). Preliminary medicine is a one year commitment which is meant to be a "preliminary" year to further training in a non-internal medicine specialty (ie Physical Medicine and Rehabilition, Diagnostic Radiology, Ophthalmology, etc). You cannot realisticaly go into practice after just one preliminary year.
Some IM programs will have a "primary care" track. This is a three year deal also. This gives an emphasis usually to outpatient medicine (and usually at the expense of inpatient experience). This does not mean you cannot do a fellowship, but it does make it harder to get into a fellowship. The idea is to give you better preparation for primary care general internal medicine (rather than practice as a hospitalist)
Have fun!
