Usually the Crop tool will take the part of the picture you have selected throw away the parts of the picture you have left out. This can be an issue when you want to access those parts later.
So now with the Crop tool you can use it normally, but Photoshop will keep the rest of the picture in the background. So it is easy to access later.
If you want to learn more about Photoshop, then why not attend one of our Adobe Photoshop Courses