There are many one man/women operations and there are large plc recruiters - and all sizes inbetween.
Recruiters are different from Internet jobs boards. Jobs boards are simply sites where individual employers and recruiters place advertisements. They are simply lists of available jobs. Some may allow you to register so that you are automatically alerted if a particular type of job is posted on the site, but other than you that have no connection/contact with the jobs board.
Recruiters are different. They will work with an employer to fill vacancies for an employer by putting forward/recommending candidates. Employers primarily use recruiters to find them suitable candidates - it saves the employers themselves having to (a) bring their vacancy to the attention of suitable candidates and (b) having to wade through possibly hundreds of applications from unsuitable candidates.
Most recruiters accept applications from individuals to be candidates with them. Good recruiters will review your situation/experience; will interview you and give you some basic careers advice.
If they take you on they will keep your CV on file, and you will automatically be contacted if any suitable vacancy comes up.
Many people register with more than one recruiter because recruiters tend to have different employers as clients. Having said that, quite often employers will send out a general request to numerous recruiters and so you may find yourself contacted by more than one recruiter about the same vacancy.
Registering as a candidate is free of charge. That's great, but it does have one major consequence - the recruiter's primary loyalty is of course going to be to the party who pays it - i.e. the employer, not you. Typically an employer will pay a successful recruiter a sum equal to a percentage of the first year's pay of the candidate placed by that recruiter. As a result, and this will not surprise you, recruiters prefer to place partners instead of paralegals because the rewards are that much greater! This is why quite a few do not yet place paralegals.