A headhunter is a recruitment specialist working for one of many agencies, ideally specialising in pharmaceutical medicine and more specifically dealing with medical roles. In general, they do not deal with entry level candidates but more at mid and senior level. However, they are working for and paid for by the recruiting company and not by the candidate (similar to estate agent who is working for the seller and not the buyer).
The quality of recruiters is on a wide spectrum –some are excellent and some are less so. It is good to develop personal relationships with a few agencies that seem to fit your needs and gain your trust. Don’t send your CV to every agency.
At entry level, it is best for you is to apply directly to companies, either in reply for advertised jobs in LinkedIn and companies’ website, specialist websites like pharmaweb and others. Ideally, if you know the hiring manager details from the adverts, write directly to them.