In the computer world, a third party may refer to either a hardware manufacturer or a software developer. It is a label given to companies that produce hardware or software for another company's product.
Third party hardware refers to components that are developed by companies besides the original computer manufacturer. For example, a person may buy a Dell computer and then upgrade it using third party components, such as an Nvidia video card and a Seagate hard drive. Since the components are not included with the computer and are purchased from companies other than Dell, they are considered third party hardware. These components would typically not supported by Dell, but instead would be supported by the original equipment manufacturer, or OEM.
Third party software refers to programs that are developed by companies other than the company that developed the computer's operating system. Therefore, any Macintosh applications that are not developed by Apple are considered third party applications. Likewise, any Windows programs developed by companies other than Microsoft are called third party programs. Since most programs are developed by companies other than Apple and Microsoft, third party applications make up the majority of software programs.
Some programs also support third party plug-ins, which add functionality to the software. For example, Adobe Photoshop supports plug-ins that add features like extra filters and selection tools to the program. These plug-ins may be created and distributed by other companies, but are designed to work with Adobe Photoshop. Therefore, they are called third party plug-ins.