Connecting Web Parts in SharePoint Portal Server 2003, Part 1 (Thx to Intranet Journal)
https://www.intranetjournal.com/articles/200408/ij_08_16_04a.html
Microsoft's SharePoint Portal Server lets different developers and organisations develop Web Parts that can communicate with each other. A Web Part is a modular unit of information that works with Windows SharePoint Services Web sites only.
Web Parts contain Web-based content and are the building block of a Web Part Page. You can also create connections between Web Parts so that when you perform an action in one Web Part, it changes the contents of another Web Part.
The SharePoint architecture supports standard Connection Interfaces for passing information among Web Parts at runtime. The connectable Web Part will act as a provider Web Part by providing information to another Web Part or as a consumer Web Part by consuming information from a provider Web Part. Two Web Parts supported by a connection interface can be connected by an end user with either Microsoft Office Front Page or an Internet browser.