Structured data describes your page in a vocabulary Google understands. It does not change what the page says to a human; it removes the guesswork about what the page is.
Formats
Google recommends JSON-LD, a script block in the head or body, separate from the visible markup. Microdata and RDFa are supported but interleave with HTML, which makes them harder to maintain.
What it can earn you
Eligibility for rich results: review stars, FAQ accordions, product prices and availability, recipe cards, event listings, breadcrumbs, job postings and more. Eligibility is not a guarantee, Google decides per query whether to show them.
It is not a ranking factor
Google is consistent on this. Structured data helps understanding and appearance. The indirect benefit is real, though: a listing with stars and a price often earns more clicks than one without.
Alien Road recommends: mark up what is genuinely on the page and nothing else. Every manual action for structured data we have seen came from markup describing something the visitor could not find, review stars with no reviews, an FAQ block with no FAQ. The markup must be a description, not a claim.