|What is a Photoshop plug-in module?
A plug-in is (also sometimes referred to as an add-on) an external program that works from inside another, so in order to use a plug-in, you have to have installed both.
What is the difference between a plug-in and a filter ?
There is no difference in theory, but I tend to use the word "plug-ins" for filters developed directly with the Plugin SDK from Adobe, which can have any interface, and the word "filter" for filters developed using the Filter Factory module (itself developed with the SDK), which provides a standardized grey interface. I also tend to use "plug-ins" for commercial filters made by companies and "filters" for the freeware work of individuals.
Where do I get them ?
If they are commercial, you can buy them at a local or online software vendor like any other piece of software. If they are freeware, you will find them on their authors' websites, or on others sites.
What is the law for plug-ins ?
The law is exactly the same for plug-ins and for other programs, i.e: if you pirate one, you can be suited just the same as if you pirate an independent application.
What do Photoshop plug-ins do ?
All sorts of things !
You have to distinguish between eight different kinds of plug-ins :
acquisition plug-ins (with the*.8ba extension) enable Photoshop to get images from external devices or programs;
format plug-ins enable you to read/write (*.8bi) images in formats usually not supported by Photoshop;
export plug-ins (*.8be) enable you to export images to other formats or devices ;
extension plug-ins (*.8bx) enable PS to take advantage of your machine, for example if you have a MMX processor;
selection plug-ins (*.8bs) are tools enabling you to make better, easier selections and masks;
parser plug-ins (*.8by) are a kind of acquisition plug-ins, only they work with vector files (such as Illustrator or EPS) ;
automation plug-ins (*.8ly), appeared with PS5, enable you to execute batch actions on a series of images ;
all other plug-ins, "effects" (*.8bf) enable you to apply special effects to your images, the same way as do the plug-ins bundled with it.