User Controls, Webparts and Visual Webparts

Visual web parts VS Standard web parts

1. A standard web part is an item-level template in Visual Studio. That means that you can only add it as an item to your project (an empty SharePoint project for example). A visual web part can be created as both item-level and project-level.
2. In the case of the visual web part, you can use a designer experience to drag and drop items on your canvas. In the case of the standard web part, you will have to manually type everything (using IntelliSense).
3. A standard web part can be deployed as a Farm or Sandboxed solution. A visual web part will have to be deployed as a Farm solution.

Web Parts vs. User Controls

One of the limitations of user control is that they cannot be shared across web applications but they can be created using a designer tool. On the other hand if you take custom controls, they can be shared across web applications, but there is no visual designer for creating custom controls through which we cannot drag and drop custom controls. By now, we understand that there is a need to have a solution that would combine the features of both the user controls and custom controls. The solution which will solve this issue is Web Parts.

Web Parts are prefabricated components that are basically targeted at easing the job of portal site / collaboration sites development. They are basically server-side controls that run inside the context of special pages (known as Web Part pages) within an ASP.NET application or a WSS (Windows SharePoint Services) site. Web parts combine the flexibility of user controls and custom controls. Developers can drag completed Web Parts from Web Part galleries and drop them into Web Part zones. It can also be said that web parts add more value over other types of ASP.NET controls in that they add extra dimensions of user customization and personalization.

User Control VS Web Part
User controls are based on Microsoft ASP.NET, and most developers are familiar with developing user controls. This improves productivity.
User controls can be used with ASP.NET -based solutions. If you decide later to convert a project containing a user control to an ASP.NET -based solution, you should be able to use the control without making any changes.
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.
User controls must be compiled before use, which adds to the time it takes to load the control.

In general, developers are not familiar with creating child controls by writing the code from scratch.
Web Parts can be used in ASP.NET -based solutions only when the solution uses Web Parts pages.
The controls in Web Parts must be added by using code.
Web Parts are pre-compiled and ready for use as soon as you need them.

In SharePoint 2010, Visual Web Parts are similar to user controls.

SharePoint 2010 Visual Web Part is an ASCX web user control that is hosted inside a standard Web Part.

Regards,
Keyur Pandya