Resizing the canvas is loss-less: it does not throw away any image information.
When you resize the canvas, the individual layers are not cropped at the edges of the image viewport; they each retain their original size, but you can just see less of them. It's still possible to move them around in relation to the viewport or to shrink them, and the image area outside the viewport will have survived. It's also possible to resize the canvas again, back to its previous size, and all the previously visible parts will have survived and be visible once more.
When you crop, any layers that extend outside the image boundary will be cropped and the outlying areas permanently discarded.