ASP.NET custom controls are more flexible than user controls. We can create a custom control that inherits from another server-side control and then extend that control. We can also share a custom control among projects.

Web Parts are components of a webpage that can be personalized and rearranged. You can use Web Parts to improve a site’s consistency while still allowing users to customize 
content to their specific needs. Forms or Windows

The ValidationSummary control allows you to summarize the error messages from all validation controls on a Web page in a single location. The summary can be displayed as a list, a bulleted list, or a single paragraph

An ASP.NET HTTP handler is the process that runs in response to a request made to an ASP.NET Web application. The most common handler is an ASP.NET page handler that processes .aspx files

A cookieis a small amount of data that you write to the client to be stored and then passed with requests to your site. You write persistent cookies to a text file on the client machine.