Selection: a selection is any highlighted area defined by a moving, broken line (“marching ants”). Selections occur when you use a selection tool (rectangular, elliptical, crop) to select parts of your image. Selections are temporary. Selections work on layers (see below).
Channel: all documents opened in Photoshop have one or more channels. A channel records information of a specific colour (eg. red, green). Most common are Bitmap (one channel in black on a transparent background which will give black on white if printed onto paper), RGB (three channels, Red, Green and Blue, which are overlaid to give other colours) and CMYK (four channels, Cyan, Magenta, Yellow and Black which are also overlaid to give other colours). CMYK is not available in Photoshop LE.
Channels can be adjusted or switched off to create different colour effects.
To switch a channel off, click the eye icon alongside the channel name in the Channel
Layers: a document usually begins with just one layer. Imagine the layer as a sheet of clear acetate with the picture printed on it. Whenever you use drawing tools or copy and paste selections, you can choose to do this on a new layer. Gradually you can build up a composite image (see page 26) of several layers. Clicking the eye icon alongside the layer’s name will hide that layer to give a different effect. Layers are a bit like collages, they are a way of building up the picture by adding more contents to it. The order of layers can be changed by dragging layers within the Layers palette to bring some items to the front or to send others to the back.
Documents containing two or more layers are larger than one-layer documents, especially when open (see page 11) so it may be a good idea to flatten (merge) all the layers into one when you have finished editing the document