What are Layers?
Within Photoshop, Layers are used to separate and organize different components of an image. This makes the whole activity of manipulating, or arranging photographic elements a lot easier.
Elements that have been partitioned on to distinct layers can be moved, removed, duplicated, mirrored, rotated, scaled, as well as other transformations and effects. All of this activity can be managed via the Layers floating palette (menu)->Window->Layers.
Layers will also generate automatically whenever you copy and paste pixels information into an image. The only time this doesn’t occur is if the files is a Bitmap or Multi-channel image. To determine this, go to (menu)->Image->Mode.
If you can’t see the Layer floating palette, then call it up. If you haven’t already done so, load an image with layers. Alternatively, make one by selecting an element with the lasso tool and copying and pasting (CTRL+C and CTRL+V) back into the same image. A new layer will have formed to retain the pasted data.
To move the contents of the layer around your image’s work-space (Canvas), you can use the Move tool (V) from the Tool palette. Ensure that you have the correct layer made active. An active layer will be darker than the rest.
Alternatively, for even better control, use Free Transform (CTRL+T) which can be called by (menu)->Edit->Free Transform. This provides the ability to change the size, rotation, perspective, mirroring and skew of your object.
Click on the New Layer icon (fig.F) to create a new transparent layer. Copying and pasting has the same effect with non-bitmap images.
Drag and drop an existing layer on to the New Layer icon (fig.F) to duplicate it. You can also do this by right-clicking (ALT+click MAC) on the layer and choose Duplicate Layer from the popup menu.
Make a layer visible or invisible by clicking on the Eye icon (fig.J).
Delete the current active layer, effect or group. You can also drag and drop it on to the Delete Layer icon (fig.G) or by right-clicking (ALT+click MAC) on the layer and choose Delete Layer from the popup menu.
To keep things tidy, it is a good idea to give you layers proper names. To do this, double-click on the name and retype.
Here is a run down of other available features on the Layers palette:
Link layers (fig.A): Link multiple layers together for moving and transforming.
Layer style (fig.B, fig.H): Add effects to your layer element such as drop shadow, bevel, stroke, etc.
Layer mask (fig.C): Create a Layer Mask so that you can mask out pixels from your layer.
Adjustment layer (fig.D): Adjust the layers lower in the stack. Alter Levels, Curves, Color Balance, etc with manageable control.
Group (fig.E): Create a group of layers for better layer management.
Layer thumbnail (fig.I): A visual representation of your layer’s contents.
Lock transparent pixels (fig.K): All transparent regions of a layer become non-receptive to the introduction of new pixels.
Lock image pixels (fig.L): Prevent a layer from receiving new pixels information. Such a layer can still be transformed.
Lock position (fig.M): Prevent a layer from being moved or transformed.
Lock all (fig.N): Lock transparent pixels, image pixels and position.
Blending mode (fig.P): Apply a blending mode to an active layer.
Opacity (fig.Q): Change the transparency/opacity of an active layer.
Fill (fig.R): Change the transparency/opacity of an active layer but keep the layer style at full opacity.
Drop down menu (fig.S): Lists a number of useful controls in relation to layers.
Saving, merging & flattening
You may want to collapse all of your layers and just work with a single layer. This can be done via (menu)->Layers->Flatten Image, or by clicking on the palette menu item (fig.S) or right-clicking and choosing the option of the same name.
You can also merge a selection of layers together. This is done by SHIFT+ALT clicking on the layers to merge so that multiple layers are active. Then (menu)->Layers->Merge Layers, or by clicking on the palette menu item (fig.S) or right-clicking and choosing the option of the same name.
If you wish to save an image file and retain the layer structure, save as either .psd or .pdf.