Wikipedia defines the practice fairly neatly and goes on to express the potential boons and pitfalls for any market participant attempting such a tactic.
It's all about public relations, dog! Mess up your public image and you invite abandonment by your customers. They can always go elsewhere for their kicks and customers ( particularly if they're consumers ) ain't the loyal little bears they once were. At least that's what all the consultancies (Boston CG, McKinsey, Bain, etc...) keep saying.
But it's a big problem, right? What do we do about big problems? Well, this guy has it figured out. Lower your quality of life now, or else!
Built-in or planned obsolescence obviously doesn't exist and Matt Tanous, an actual engineer, explains why the concept is a fantasy.
In short, reputation on the one hand plus actual technology considerations on the other. The reputation stuff goes back to what's in the Wiki article, but the tech stuff concerns increasing complexity of electronics and the compacting of ever more components into the same space in every successive generation of, say, iphones.