A marketer does many things, but primarily - promotes products for either themselves or another company, for either sale or information (depending on what their brief was in the first place).
If they are promoting products for sale, they will find out their target audience (the market), and tailor a sales campaign or ad to suit. If they are doing it for information for the client then they will find out what the target audience is likely to respond to and adjusts their ads / campaigns to suit. They get paid in varying ways, either a commission per sale, or a flat fee for the promotion. If they are promoting products for themselves, then the sky is the limit for the mount of money they can earn. There is varying degrees of work that you would have to put in to each project, some a small (create a small text ad), or large, create a whole video campaign and market / promote it to many different avenues for the target audience to see. There is tonnes of marketing info at marketing websites (even check out marketers profiles including ours). Hope that helps.