Many of the books and podcasts on Photoshop will always have the term Key Board Shortcut in it. What these are are keyboard combination to do is perform certain functions like copy and paste. For a lot of people just starting in Photoshop it is enough to remember where the commands are in the menus let alone obscure key combos.
While helpful they can be daunting. There are over 278 different official keyboard shortcuts and another two dozen that seem to pop up at will. Some of these seem to have more fingers and functions than on a human hand. This including the undocumented Ctrl + Alt + Shift + K (for Mac Cmd + Opt + Shift + K) which brings up the key board short cut menu in Photoshop.
However I do use about a dozen KBS (Keyboard Shortcuts though you would think the abbreviation would have been shorten to just KS) which are helpful in everyday Photoshop operations. If you only play in Photoshop once a month then don't worry but if you are working in it everyday then they can be time saver.
Ctrl + Z (Mac Cmd + Z) for a one step undo. The most helpful KBS of all and functions in other programs too. Add Alt or Option with this and it will step back about 20 times.
Ctrl + C (Mac Cmd + C) for copy. When you an image or text selected this KBS will copy the contents to clipboard which runs in the background. Will stay there untill you copy something else or turn off the computer.
Ctrl + V (Mac Cmd + V) for paste. Yes I know why isn't this Ctrl + P for paste but that is setup for printing. So I quess it stands for Very Sticky.
All three of these KBS will also work in word processing programs and some other graphic programs too.
So now for some of the Photoshop centric KBS which are helpful.
Ctrl + J (MAc Cmd + J) for new layer. With nothing selected this will make a new blank layer above the layer you are currently on. Here is what is cool, if you do have selection made this KBS will copy that selection to a new layer. Then you can just start working on only it for Transforming or special effects.
And speaking of Transforming Ctrl + T (Mac Cmd + T) will give you a free transform box around your selection. This is when you see a box around your selection with the eight handles, four in the corners and four on each side. Clicking on these boxes will let you shrink or skew your image. To keep the image in the correct ratio hold down the Shift key and move the corners. Add the Alt or Option key and the box will move toward the center.
One last one for Layers, it is a handful but only on the left side of the keyboard. Ctrl + Alt + Shift + E (Mac Cmd + Opt + Shift + E) will make a merged copy of all the layers without flattening the layers. What this makes helpful is for those times when you need a flatten copy of all the layers but still want all the layers in case you want to go back and re-adjust or want to click and drag this layer to a new image. Go to the top of your layers and do the left hand keyboard shuffle of these keys and it is done. Now with this new merged layer you can apply sharpening to it and change the mode to Luminosity. By over sharpening this layer with the mode change will reduce the halo effect and a reduction of the opacity to taste gives you more control over the look. You can also add a layer mask and bring back some of the original softness.
One more I promise, Using the ALT or Option key when clicking on the Layer Mask Icon at the bottom of the Layer Pallet. This will make a Layer Mask but it will filled with Black to hide the effect instead of White which shows everything. Lets say you copied the whole image into a merged layer and applied a soften filter to it for the smooth skin look. The soften filter will soften everything and not just the skin. Well with an all black layer mask this effect is hidden. Now with a soft edge brush and the color white you can paint the soften look on just the skin. Plus you can play with the opacity of the layer for the best look.