iOS Difference between UIView's drawRect and CALayer?

Think of UIView as a wrapper around CALayer. Each UIView has one CALayer backing it. But a CALayer isn't necessarily associated with a UIView.

Every UIView has a CALayer. It's what a drawRect draws into.

UIViews, which encapsulate a bunch of stuff, including a built-in CALayer and event responder (etc.), are usually easier to use than all the sub-parts individually.

But using CALayers directly has the advantage that you can draw, swap, change, and update them in background threads (not so with most UI objects), and thus not have to wait for a drawRect callback, nor slow down the responsiveness of the app's UI main run loop with any lengthly draw.