When editing in Photoshop, you have the option of using nondestructive editing techniques, which means that you are not necessarily changing the original image. This not only allows you to go back to the original image at any time, but also allows you to save the image in its original state. The key to this process is layering.
Each image you open in Photoshop starts with a single layer. You can create additional layers of that image to edit -- this is where the term nondestructive editing comes in. Editing the copy of the original image will safeguard against any mistakes or alterations you saved but didn't like the next day. This is also where you can get into more fancy processes, like cropping portions of an image out and pasting them on a new background, or changing the tone and color gradient for certain portions of an image while maintaining the original for a particular area. If you want to merge more than one image together, the layers panel is where you would set everything up. Once you are familiar with layers, check out the tutorials Basic Layering and Basic Masking below to get into some more advanced edits.
Breaks down what layers are and how to use them effectively.
More information can be found on Adobe's layer tutorial.
Once you have layers down, you can use them to create masks to apply to images. Layer masks are incredibly useful to maintaining a nondestructive workflow. They allow you to reveal parts of the layer underneath, without directly editing the image on the layer above.
See the video below to understand how layer masks work, then try it yourself with this step-by-step layer masking tutorial by Adobe.