Probably the biggest factor that differentiates recruitment firms from headhunter agencies is that the former charges contingency fees that is generally payable only if the agency is actually able to find the right candidate for the vacant position. On the other hand, latter one is paid a part of the commission regardless of the fact whether they are able to find the ideal candidates for the position.
Apart from that, the quality of services one gets to redeem from these two agencies can vary greatly and can be quite different from each other. Needless to say, recruitment firms do make it a point to be careful of their repute, and make sure that only the most suitable candidates are referred to the organizations. But their success is also dependent on the number of candidate referred. More the number of vacancies they are able to fill, higher they will be paid. That is why; sometime number of referred candidates takes priority over the quality. That is why recruiting firms cannot necessarily refer you the best candidates.
Contrary to this, headhunter agencies are paid at least a certain amount of money, even if they may not be able to find to the most suited candidates. Their reputation comes from the fact that they are more concerned about the quality of candidate rather than quantity of candidates. This is the reason they are more vigilant about the work culture and the staffing requirement of their clients, so as to be able to find ideal candidates. That is the reason; headhunter agencies are preferred by organizations.