The crop tool can be used to crop an image or layer. To get started, choose the Crop tool and then click once to select the entire canvas. Using this method you ensure that you don't inadvertently grab outside of the visible canvas as part of the crop. You can then use the options below to refine your crop.
At its most basic, the crop tool allows you to size a rectangle around an area and reduce your image or layer to only that content which is contained within that area. There are several options which give a bit more flexibility and precision.
The two numbers on the left are the exact horizontal position and vertical position of the left and top of the cropping frame respectively. The numbers are the right are from top to bottom: width, height, and aspect ratio. Selecting the check boxes will keep any one of these can be locked to allow you to manipulate the other two without losing the position or ratio of the locked property.
If you crop an image, and try to start a new one directly afterwards, Krita will attempt to recall the previous crop, so you can continue it. This is the continuous crop. You can press esc to cancel this and crop anew.