Crop Tool

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.