An action is a programmed set of commands that you can use, create and apply within Photoshop. Photoshop comes with some preprogrammed actions. Go to the Actions Palette and you can view what actions are already there. Simply open the folders and take a look.
You can run an action by selecting an action and pressing the right facing arrow in the actions palette. This will "play" the programmed action. If you select "wood frame" for example, the action will run and create the commands necessary to end up with a wood frame.
The real power of actions is that you can create your own recordable actions. For example you can create your own folder, new action and press record. Then you would just go through whatever actions you are taking in Photoshop and it will record all of the different settings and everything. When you are done, you can apply this action to other layers or documents.
Examples of Actions that you could record are; image adjustments, color balance, image size (an important one), creating frames or borders, the list goes on! Think of an action as something that you can record that you would apply to other documents to get rid of tedious tasks. You can open another document and go to the Actions palette and run the action and it will apply to the new document.
If you notice that you are doing the same commands for your uploaded or scanned in images (ie. levels, color balance..), just record an action and then run it on each image or automate the task. Actions is an advanced tool of Photoshop that can really help to streamline your workflow. You can find some really good actions out there or create your own.
You can even use the File: Automate: Batch command and run an action across an entire set of files (ie. open files or a certain folder). When running pre-programmed actions, they will usually create a snapshot of your document before it makes the changes so you can go back to the original state. Consider making snapshots on your actions.
I recommend ActionFX.com by Al Ward. He has some really cool actions that create some great special effects and text. LumiQuest.com also has actions which you can load and run for image improvement. They are the leader in soft lighting for digital and film. Actions such as image enhancements for digital cameras are quite common.
