Microsoft SharePoint Designer (SPD), formerly known as Microsoft Office SharePoint Designer, is a specialized HTML editor and web design freeware for creating or modifying Microsoft SharePoint sites and web pages. It is a part of Microsoft SharePoint family of products. It was formerly a part of Microsoft Office 2007 families of products, but was not included in any of the Microsoft Office suites.
Questions having the sharepoint-designer tag regards the use of SharePoint Designer in every sense, such as configaration, development and design of SharePoint sites, pages and lists.
Reference: Wikipedia - Microsoft SharePoint Designer