Typically they'll post jobs on their website or through employment search engines such as Indeed, Monster and Careerbuilder. They'll call you in for an interview and will test your skills. They'll give you interview tips and access to their skill development online classes. They'll promise to submit you for a job or two and boom you never hear from them again.
While headhunters and placement agencies can help you get a foot in the door especially for temp and temp to hire roles they're notorious at leading candidates on and for not following professional communication etiquette. They'll send you on interviews and never call you again. They'll ignore multiple follow up attempts and leave you wondering what the heck happened.
So are corporate headhunters and agency recruiters worth the headache? Is it better to use them instead of applying to a company directly? Do the benefits of working with corporate headhunters and recruiters outweigh the cons? While it depends most of the time I would say yes to the proceeding questions. There have been companies that I applied to a couple dozen times with no luck. As soon as the headhunter made the introduction I got the interview.