Offset printing transfers ink from a custom-made plate onto paper, producing precise, consistent color at a lower per-unit cost — but only once a plate has been made, which involves a setup cost and time that makes it efficient mainly for large print runs.
Digital printing prints directly from a digital file with no plate required, making it faster to set up and cost-effective for small runs — but generally with a slightly higher per-unit cost at scale and marginally less color precision than offset.
As a rough guide: digital printing suits smaller runs (a few hundred cards, a short-run brochure), while offset becomes more economical once quantities scale into the thousands.