The Crop Tool cuts out a defined
area deleting or hiding the rest of an image:
In the Toolbox, select the Crop Tool, click on the image and draw a rectangle holding mouse's button
pressed.
When you release the button a bounding box appears.
You can move, resize and rotate it. To move bounding box by
a few pixels you can use keyboard cursor keys.
While bounding box is active
you can choose Hide or Delete option: Delete completely remove cut-off (shielded)
parts of the image; Hide only hides them, so
you can make them visible later on using Move Tool..
To apply crop you have either
to press [Enter] key or right-click on the image
and choose command Crop in context menu.
To pre-define target image size fill Width, Height and Resolution fields
on the Options bar. Units (cm or inch)
have to be specified in the same fields; otherwise Photoshop
sets it to px (pixels).
Note that while bounding box
is active the rest of Photoshop tools and commands are not available.
You have either to apply [Enter] or to discard
[Esc] crop to resume the work.
*Transformations are discussed in the 2nd chapter of this video-course.