1.difference between visual webpart and a standard webpart'
the SharePoint 2010 Visual Web Part is an ASCX web user control that is hosted inside a standard Web Part.

2.Difference between web user control and webpart
user control:
a.User controls must be compiled before use, which adds to the time it takes to load the control.
b.The Visual Web Developer designer provides support for designing the UI by using drag-and-drop operations that gives the control a consistent look-and-feel, which results in faster development.
Webpart:
a.Web Parts are precompiled and ready for use as soon as you need them.
b.The controls in Web Parts must be added by using code.

3.SharePoint site pages vs application pages
Site Pages:- Site Pages are Site or Web Scoped, User can customize Site Pages, Site Pages are stored in Content Database,
You cannot have custom code in Site Pages, Site Pages are Un-ghosted Pages.

Application Page:-
Application Pages are farm scoped, No customization can be done by the user, Application Pages are stored in WFE(Web Front End) in _layouts folder, You can have custom code in your Application Pages, Application Pages are Ghosted Pages.

4.Difference between WFE and application server
Microsoft SharePoint Foundation Web Application makes WFE in SharePoint farm

5.Difference between page library and site library
The Pages library is for storing Publishing Pages whereas the Site Pages library is for storing wiki and web part pages.
You only get the Pages library once the SharePoint Server Publishing feature is activated on the site. This library is usually used when you have requirements for content-centric pages that may use the scheduling, workflow and caching capabilities of SharePoint; such as in the case of public-facing websites. In a more collaborative scenario, you'd want to use the Site Pages library to create wiki and web part pages.