The query string is a collection of key–value pairs, separated by an ampersand (&) character. The start of the query string is indicated by a question mark (?). 
The following provides an example.
GET /getCustomer.aspx?Id=123&color=blue

An ASP.NET theme is a collection of styles, property settings, and graphics that define the appearance of pages and controls on your website. A theme can include skin files, which define property settings for ASP.NET web server controls; cascading style sheet (.css) filesthat define colors, graphics, the size and placement of controls.

A user control is a file we create that contains a set of other ASP.NET controls and code grouped together to provide common functionality. The user control can then be used on different pages within a website.

A web service is a web application which is basically a class consisting of methods that could be used by other applications.