Jobs to Be Done is the theory that people buy products to make progress in a specific circumstance, not because of who they are demographically. Clayton Christensen and his co-authors stated it plainly in Harvard Business Review: when we buy a product, we essentially hire it to help us do a job, and if it does the job well, we hire it again [1]. The job is the progress a person is trying to make, the circumstance is the context that shapes it, and the alternatives are everything else that person could hire to make the same progress [1].
The canonical illustration is Christensen's milkshake study. A fast-food chain found that nearly half its milkshakes sold before mid-morning to solo commuters, who hired the shake over a bagel or a doughnut because it was tidy, it staved off hunger until lunch, and it gave a bored driver something to do on a long commute [2]. Demographic segmentation would never have surfaced that pattern, because the real competitor set was defined by the situation, a long boring drive plus looming late-morning hunger, and not by the buyer's age or income [2]. Two customers with identical profiles hired the milkshake for entirely different jobs at different times of day, which is the point: the job, not the demographic, predicts the purchase [1][2].
Strategyn, the consultancy Tony Ulwick founded to operationalize the theory as outcome-driven innovation, defines a job as the task, goal, or objective a customer is trying to accomplish in a given situation, and ties customer-defined outcome metrics to that job so progress becomes measurable [3]. Christensen's article adds the dimension websites most often miss: a job is never purely functional, it carries social and emotional weight, how the buyer wants to be perceived and how they want to feel [1]. For a strategy stage, that is the useful part. If you know the functional job, the social job, and the emotional job a visitor is trying to get done, you know what every page has to accomplish before you write a single headline [1][3].