Using the crop tool in Adobe Photoshop CS©

• To cancel a crop, hit the escape key. To apply the crop, hit the enter key, double click inside of the crop bounding box, or choose another tool from the tool bar.
• To use the Crop tool to add canvas to your image, drag out a crop and release the mouse. Then, drag the crop handles outside of the image area and apply the crop. The area outside of the image will be added to the canvas. Note: to add transparency around the image (instead of filling the added space with the background color) convert the background into a layer (by double clicking on Background in the Layers palette).
  To eliminate any information that is outside or beyond the visible image area (and possibly decrease file size), choose Select All > Image > Crop.
• To use the dimensions of one image to crop another, select the document with the desired crop (width, height, and resolution) and click the "front image" button in the Options bar. This will enter the height, width and resolution for the document in the Options bar. Then, switch to the document that needs to be cropped and use the crop tool. The aspect ration will be constrained while dragging the crop and, when applied, the image will be resized to the desired width, height and resolution. The forward slash key toggles on and off the shields while using the crop tool. To temporarily override the "snap to" options, hold the control key while dragging.
• To make a crop that is a specific size and resolution in option bar enter the desired width and height and resolution/dpi. Now drag the crop tool in your image. This will make a crop that is the that is the exact size you specified while still allowing you to choose the area to be cropped. Click on clear to erase the specified dimensions and resume using the crop tool as usual.