Adjustment Layers

©Doug Weldon Photography -
You will note that most of the adjustments made to images in these tutorials use adjustment layers. While the use of layers seems to complicate the editing process, it in fact, simplifies it. While a complete discussion of adjustment layers lies outside of the of these tutorials, you are asked to "trust me." Adjustment layers are the way to go.
All techniques discussed in these tutorials can be accomplished without using adjustment layers. In fact, you will see significant improvement of your images by applying the adjustments discussed directly to your images. However, by not using layers, you will be directly manipulating the image's pixels, forever changing the image and impacting its final quality. Using Adjustment Layers eliminates these irreversible changes, makes your editing task much more flexible, and results in a higher quality final image. 
Now, some will argue that the History Palette allows one to "back up" and eliminate unwanted changes made to an image. This is true to some extent, but using the History Palette has distinct limitations that the use of layers overcomes. The History Palette, while useful, has the drawback of eliminating changes in gross fashion. That is, if you have a series of changes recorded in the History Palette, you can reverse only the last one without affecting others. If you wish to reverse an early change in the list of changes, you lose all other changes that follow it. That's a pretty big drawback! That and the fact that each edit directly applied to the image's pixels further degrades the image.
The use of Adjustment Layers allows you, not only to impact individual edits you have made to the image, but to do so without affecting others. An additional benefit of the use of Adjustment Layers is that you can adjust the "degree" or "strength" of an edit that you have made by using the Opacity control for the layer. Still another benefit (for the advanced user) is that, through the use of layers and masks, adjustments can be applied to select areas of an image. Masks become a very powerful tool in editing nature images.
The Layers Palette can be displayed in your editing environment by choosing "Layers" from the "Window" menu.
All adjustments discussed can be easily accessed in the Layers Palette by clicking on the black and white circle at the bottom of the palette. This is the Adjustment Layer Tool.
Feel free to move forward in the tutorials applying all edits discussed directly to your image, if you wish. You will, however, be missing out on a powerful and very flexible tool that should become a staple in your arsenal of digital editing tools.
It should be noted that I depart from using layers for sharpening and noise reduction in the introductory tutorials, as sharpening and noise reduction layers do add a bit of complication for the Photoshop novice. The use of layers for these two adjustments is covered in the advanced tutorials.