In the context of digital photography, a "plug-in" is a small software program which is used by another software program, such as Adobe Photoshop, to perform some task. For example, Photoshop can handle and process a multitude of image formats, such as JPEG, TIFF, PSD, etc. However, many digital cameras, while able to handle some of these general formats, also use a "raw" format. The raw format is a proprietary format that represents a (mostly) unprocessed image. In order for Photoshop and other image processing programs to be able to handle these raw images, the camera manufacturers make plug-ins available. These plug-ins provide the interface between Photoshop and the image. In general, the raw image plug-ins are available at no charge. Adobe Photoshop users can periodically download and install new updates to the "Adobe Camera Raw" module, which is a collection of plug-ins for various cameras. Additionally, camera manufacturers may make their raw plug-ins available on their website for users to download.
Other types of plug-ins are also available. For example, many Photoshop filters, such as sharpening filters, are implemented as plug-ins. This type of plug-in is generally not free, but is sold as a product.