A plug-in is a small software component that adds new features to another program. They may add new functionality, modify the user interface, or allow the software to support additional file formats. First-party plug-ins may be packaged with an application, while third-party plug-ins can be made available as a free download or as separate commercial software.
Many professional graphic design, video editing, and digital audio workstation applications include support for plug-ins. For example, a plug-in for Photoshop may add new filters and editing tools, while a plug-in for ProTools may add new synthesized instruments and audio effects. Media players use plug-ins to add support for new file formats and codecs.
Web browsers use a type of plug-in called extensions to add new features. Unlike plug-ins for other types of applications, web browser extensions are distributed as source code instead of compiled software. Historically, web browser plug-ins like Adobe Flash distributed as executable software could lead to security vulnerabilities and software instability, so browser developers dropped plug-in support in favor of extensions. However, the purpose behind them is the same — to allow the user to customize the appearance and functionality of a web browser as they like.