A VSIX package is a .vsix file that contains one or more Visual Studio extensions (VS2010 onwards).
A VSIX package is a .vsix file that contains one or more Visual Studio extensions, together with the metadata that Extension Manager uses to classify and install the extensions.
That metadata is contained in the VSIX manifest and the [Content_Types].xml file. A VSIX package may also contain one or more Extension.vsixlangpack files to provide localized Setup text, and may contain additional VSIX packages to install dependencies.
Templates are provided in the Visual Studio SDK to enable the creation of vsix packages.