What do you mean by WSP?

It is not file extension of web part. once we implemented the code we can pack that as wsp and deploy. that code contains List,Webpart,eventreceivers, Masterpage,...

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.