Controls how flat a surface must be. No datums needed — it's self-contained, making it one of the simplest form tolerances to apply.
Controls the entire cylindrical surface at once — roundness, straightness, and taper combined. Stricter than circularity, and harder to inspect.
Controls how round a cross-section must be at any given slice. Applied independently at each cut — it doesn't control the overall cylinder.
Controls how straight a line element or axis must be. Can apply to a surface line or to an axis — and the difference matters more than most people realize.