Headhunters, also known by the somewhat more dignified name of third-party recruiters, are executive search firms that specialize in filling mid- and high-level jobs at organizations ranging from giant corporations to small not-for-profits.

As a profession in the United States, headhunting dates back at least to the 1920s, but the field seems to have really taken off in the go-go years of the 1950s and '60s. Today it is a global business taking in billions of dollars in fees annually.

Corporations and other kinds of organizations can fill their executive vacancies in an assortment of ways. They may have full-time recruiters in their own human res departments who search for suitable candidates both within the organization and on the outside. Or, they may out that job to an independent executive search firm. These independent firms are what most people think of as headhunters.

Executive search firms can be large, international businesses with hundreds of consultants or small boutiques with just a handful of recruiters. Some firms are generalists, assisting in searches across a wide variety of fields. Others specialize in a certain industry, such as healthcare, information technology or retailing.

There are two basic types of firms (and consultants); the major distinction between them lies in how they are paid:

Contingency recruiters search for prospects with the understanding that they'll be paid only if they are successful in filling the job. Their commission is usually a percentage of the position's annual salary. A company might use more than one contingency recruiter at a time for a particular position.

Retained recruiters are paid a retainer to conduct a search and screen potential candidates. Like contingency recruiters, their compensation may be based on the position's salary, but it doesn't depend on whether the job is ultimately filled. Their contracts with employers also tend to be exclusive, so, unlike contingency recruiters, they aren't competing against other recruiters to fill a position.

As a general rule, retained recruiters are used for higher-level searches involving larger salaries – often a base of $250,000 or more, according to the Association of Executive Search and Leadership Consultants, a trade organization representing retained search firms in the U.S. and other countries.

Some search firms also offer a hybrid approach, often referred to as a container or retingency search. In that case, the firm receives some money up front, with the rest contingent on a successful job placement.

Whatever their compensation arrangement, headhunters are paid by their clients, the organizations that hire them, and that is where their loyalty lies. They are not paid by job candidates, and as a result, recruiters' obligations to job hunters are limited.

If you're a candidate, that means you shouldn't expect too much in the way of career counseling, resume critiquing or soothing advice from a headhunter. Don't be surprised – and try not to be offended – if at some point the recruiter stops returning your calls or answering your emails. Simply assume that, for whatever reason, you just weren't a match for that particular assignment.

Headhunters start with a job description from the client company, detailing what it is looking for in the new hire. Then they set out to find candidates who match that description as closely as possible.

One place they will look is in their firm's database, which is a good reason to make sure the headhunters in your field have your resume and that you let them know when you change jobs or add a major new credential, such as a degree or a professional certificate.

They will also reach out to their contacts in the field, such as executives they have previously placed, for word-of-mouth referrals. If all else fails, they may simply cold call or email potential candidates whose names they have obtained from association directories, LinkedIn, Facebook or other s. According to a 2014 survey by the website Jobvite, 94% of recruiters reported using LinkedIn, while 65% used Facebook.

If you are in the market for a new position, it's essential to make yourself visible in social media venues like these. The easier it is for a headhunter to find you, the more likely that one eventually will.

Bear in mind, too, that any negative information a headhunter might see online could sink your chances of recruitment. A 2012 survey by the website ExecuNet reported that 90% of recruiters type potential job candidates' names into a search engine to see what comes up. Evidence of a criminal record, accusations of sexual harassment or Facebook photos showing unprofessional behavior were among the grounds for scratching candidates off the list, the survey reported.

Similarly, the 2014 Jobvite survey found that 65% of recruiters reacted negatively to profanity and 61% to grammar or punctuation errors.

Once they have a list of names, headhunters will conduct a first interview to assess each candidate and begin to narrow the field. To save time, that first interview may be done by phone. Candidates who pass this initial screening are likely to be invited to an in-person interview.

Ultimately, the headhunter will distill the list to a handful of candidates to be presented to the client. The client will then decide which, if any, of the candidates to interview and, ultimately, whom to hire. If none of the prospects seems exactly right, perhaps because the nature of the position has changed or the client has had further thoughts about the kind of person he or she wants in that job, the headhunter may start all over again to come up with a new list.

Headhunters are one of a number of ways that organizations look for talent, especially for the top executive ranks and in hot fields where the demand for workers exceeds the supply. Making yourself visible to headhunters can be a smart career move, and getting a call from one can be flattering. Keep in mind, however, that the recruiter's first responsibility is to the client company and his/her own commission. Your hopes, dreams and career aspirations are likely to be lower on the priority list.

For more on conducting your own job search, see The Complete Guide to Job Searching and The Best Online Job Search Techniques.