There is more to just making your photo smaller and trimming away unwanted edges to the Crop Tool. You can also add more pixel, align, match dimension of another file etc…
You can access the Crop Tool by using the shortcut C or go to the tool bar and click on the crop tool
The most common action is to crop out unwanted edges, drag from the upper left corner to the lower right corner. You still have a chance to make any final tweaking by moving the crop handles or clicking inside of the crop area and move it around. If you are satisfied with the crop, either double click inside the crop area or hit Enter. Pressing ESC key will cancel the crop operation.
You can also crop beyond your canvas thus adding more pixels to your file. First draw the crop mark anywhere and any size is fine, pull the handle and drag it outward beyond the canvas, when you hit enter you will notice new areas are added to your canvas.
Is your photo crooked? You can straighten the photo so that it is perfectly parallel with canvas or intentionally make a photo crooked by rotating the crop handles. Place your mouse near the out side of the crop mark, you will notice your mouse turns into a semi circle with arrow.
If you need to crop your photo down to a specific size you can set the Width and height in the crop menu.
You have the option to change the resolution by entering the new value in the resolution box. Leave it blank if you do not want the resolution to change.
To and resolution you can click on the file you want to reference from and click on the Front Image button. It will fill in the Width, Height and Resolution in the menu for you, now draw a crop mark on the file you want to match it to.
You do not need to use all three value once you click on the Front Image button, you can choose to keep just one or two of the value by deleting the rest of the values.
More option will appear once the crop is drawn.
You can choose to Delete or Hide the cropped area. If delete is selected anything that was cropped out will no long be available even if you have a layer. If you try to drag any layer you will notice that anything beyond the cropped area has been deleted as well. If hide is chosen, the information beyond the cropped mark will still be available for editing later.
Shield Color and Opacity will help cover up the potential area to be cropped out to help you better visualize what the final image will look like.
Perspective check box will allow you to distort the shape of the crop mark. This can help correct some minor distortion problem due to perspective. This is not available if fixed length is selected earlier.
You can also crop with any selection tool by making your selection first than go to Image-Crop. If your selection is not a rectangle than it will just take the very edge of your selections as the boundary of the crop.
Hold Ctrl/Command to temporary disable snapping.
Hold Shift to create a perfect square.
Holding Alt/Option will make let you draw the crop mark from inside out.
To keep the same aspect ratio, first draw a crop to fit the canvas, than hold Shift while cropping down to retain the ratio of your crop mark.
I hope this tutorial was useful, if you know of any other tricks related to crop please post that in my comment. Let me know which part of this tutorial you thought was the most useful.