its the the file extension of a web part.

- WSP stands for Web Solution Package. It is a cabinet file that contains assemblies, resource files, features, images, application pages, site definitions etc. into single file. - Whenever any customization or upgradation or updation is carried out in SharePoint, various items like assemblies, resource files, features, images, application pages, site definitions etc. are required to deployed. - It becomes difficult to deploy these items separately in development, staging and production. This called for a powerful deployment tool in the form of WSP.