Question regarding "Actions" and "Plug-ins"

Regarding Photoshop.
What is the difference between "actions" and a "Plug-in"?

Can some recomendations be made for a few of each.

This came to mind while I was researching On-One Focal Point software, I was recommended to look for free actions.

actions are basically automated steps of things that you do often in Photoshop, or steps that you wouldn't remember to repeat again to get the same results time after time. (ie. shrinking a photo to a set size, adding a border, a watermark and then saving to a directory before closing)

plug-ins are little applications that have been written to produce an effect within Photoshop. ie. Unsharp Mask to sharpen a photograph.

so you could have an action that runs 1 or more plug-ins..