Recruitment agencies are outside firms who go and find candidates for employers.
Do not confuse recruitment agencies with employment agencies/labour brokers. The main difference lies in who ends up being your employer.
The most common misconception about recruitment agencies is the idea that recruitment agencies find candidates work. In most cases, recruitment agencies do not find jobs for people, rather they find people to fill the positions that their clients are wanting to fill.
Recruiters can be incredibly useful in helping you with your job search. All you have to do is be a great candidate and follow their guidelines and advice!