There are so many different types of Marketing jobs, so there's no one right answer. Most Marketers will have to be responsible, in some capacity, for at least these things:
Recognizing what people like and don't like
Determining what people buy and won't buy and why
Public relations, blog content, brochure editing, digital marketing efforts, market and consumer research to understanding marketplace/industry trends, translating those trends into consumer insights and business implications, analyzing communication, usage, brand loyalty and media, just to name a few areas.
If you work for an agency, such as a marketing or advertising agency, on the other hand, you may touch many different brands. Nearly every industry, service or product, has a marketing component. The direction you may pursue may also depend on where your strengths and passions lie. Are you, for instance, more interested in research? writing? sales? promotions? Are you good at presenting? Turning data into a compelling story? graphics and art? As you can see, there are many options.
The great news is that is a vast industry with room for many different types of talent and aptitudes. I, for instance, have been in the "marketing" world for nearly 20 years, yet have worked for advertising agencies, media and consumer trend insight companies -- all which have heavy marketing components, but aren't pure marketing organizations or jobs. As an editor, for instance, I was responsible for turning marketplace trends into stories that would help other marketers form better relationships with their consumers.
You may want to spend a little time online researching different types of marketing jobs. BLS.gov may be a good place to start: https://www.bls.gov/ooh/business-and-financial/market-research-analysts.htm or https://www.bls.gov/ooh/management/advertising-promotions-and-marketing-managers.htm
Another thing you may want to do is do search on job boards like Indeed.com or Monster.com to see what types of duties and responsibilities are posted under job descriptions for marketing positions.