InfoPath form is the SharePoint feature provided to create form without any HTML and CSS code. InfoPath forms are created by the InfoPath Designer tool.
SharePoint Form library is the collection of XML forms (InfoPath forms). SharePoint Form library available in the Site Contents of the site used to create/save/update the InfoPath forms.
C# was developed by Microsoft. It is component-oriented language and was first released in 2002. This language has grown and changed across seven major versions.