@Tes yeux noirs,
In my area of the western U.S., a fellow was someone, man or woman, who got financial help while continuing to climb the steps of academia. They were generally what we call residents in medicine. Past internship, often for years/ That is not a negative, except that hours can be long.
A professor is a step above or quite above, in seniority. In my memory, most of those folks had passed lengthy board exams, and been published, usually more than once, depending on whether or not they were first author and so on.
Don't trust me, things may be different elsewhere or different from when I knew it.