Updated for Xcode 12.5

SwiftUI gives us five built-in shapes that are commonly used: rectangle, rounded rectangle, circle, ellipse, and capsule. The last three in particular are subtly different in how they behave based on what sizes you provide, but we can demonstrate all the options with a single example:

struct ContentView: View {
    var body: some View {
        ZStack {
                .frame(width: 200, height: 200)

            RoundedRectangle(cornerRadius: 25, style: .continuous)
                .frame(width: 200, height: 200)

                .frame(width: 100, height: 50)

                .frame(width: 100, height: 50)

                .frame(width: 100, height: 50)

That draws all five shapes: two at 200x200 and three at 100x50. However, because the drawing behavior of the shapes is different you’ll see all five shapes visible in the output:

Rectangle draws a box at the exact dimensions you specify.
RoundedRectangle does the same, except now you can round the corners by a certain amount. Its second parameter, style, determines whether you want classic rounded corners (.circular) or Apple’s slightly smoother alternative (.continuous).
Capsule draws a box where one edge axis is rounded fully, depending on whether the height or width is largest. Our shape is 100x50, so it will have rounded left and right edges while being straight on the top and bottom edges.
Ellipse draws an ellipse at the exact dimensions you specify.
Circle draws an ellipse where the height and width are equal, so when we provide 100x50 for the space we’ll actually get 50x50.