1. This is a full width section with a fixed height. If you make the window narrower or wider it always fills the page from left to right. The height always stays the same and the image fits inside whatever space it has. This leads to some cropping unless the section ends up with the same dimensions as the original image.

2. This is a fixed width section. On wider screens it will display with the border on the side.

3. This is a full width image. The dimensions are retained which means none of the image is cropped. However it will display very differently on different size screens.