Layers are one of the most useful features in Photoshop. Layers are a relatively simple concept that increases the power and flexibility of building images much easier. Through the use of layers Photoshop allows you to work separately on the various component parts of your image.
So what exactly are Photoshop layers? In fact layers are independent images that can be manipulated in Photoshop just the same way that any graphics program can manipulate any image. The only difference is that each layer really only forms a part of the final image. The final image will be made up from all of the layers within the image depending on the order tat the layers are in and the transparency levels that are set for each layer. You can work on each layer as if it were an independent image without interference from any graphical elements that are on other layers.
Every time that you paste anything into a Photoshop image a new layer is created containing the new content so creating new layers in your image is automatic. You can also create additional layers any time that you want from the Layer menu by selecting New and then Layer.... When you do this the following dialog box will appear allowing you to create your new layer.
The dialog box allows you to set the initial opacity of the layer which controls how much of lower layers will show through. Of course this does not apply to any parts of the layer that are defined as transparent where all of the lower layers will show through regardless of the opacity setting. Opacity can be used to create interesting effects (as can selecting a mode other than Normal when creating a new layer). Changing the opacity to allow other layers to show through is also useful when it comes to aligning elements to exactly the position you want, you can always change the opacity back once your finished. You change the opacity or mode after creating a layer using the layer palette, just click on the layer that you want to change and change the values at the top of the palette.
In fact, you can do almost all of the manipulations of the layers in your image using the layer palette.
The only layer manipulations that can't be done using the layer palette are merging of layers. These options are available via the Layers menu which has three options to do this at the bottom of the menu. These options are Merge Linked which will merge the layers of a clipping group into one layer, Merge Visible which will merge all of the visible layers into one layer, and Flatten Image which will merge all of the layers into one layer allowing you to then save your image in a variety of different formats that don't support layers.
Layers are a very useful option so you may want to consider storing the master copy of all of your images in Photoshop's proprietary PSD format which allows layer information to be preserved. You can then edit the image as required and flatten it only when you are ready to create a new version of the image that you wish to use in other programs (or on the internet). In many cases this flattening wont be necessary because the program where you intend to use the image may be able to use the PSD version directly.
This is only a brief introduction to Photoshop Layers. There are many more things that can be done much easier when you use layers to keep the components of your image separate. Why not experiment and see what you can achieve using Photoshop Layers. For an example of one image I created using layers see the page Manipulating Layers.
This article written by Stephen Chapman, Felgall Pty Ltd.