Is there any difference between Postdoctoral fellow and research associate?
This will vary by institution.
Post-doctoral fellow, at least in the US, is almost always a temporary position to bridge between grad school and a faculty position. Some funding agencies (and therefore some universities) require that post-docs received their PhDs within the past 7 years.
Research associate can mean almost anything. At MIT, senior research associates:
"possess a demonstrated research competence equal to that of a tenured faculty member...Appointments are of indefinite duration and are to be reviewed within every five-year period"
So, a potentially permanent, but untenured research position with qualifications parallel to the tenure track. In other contexts, this can be a temporary position granted at any level.