Introduction to Web Parts

Web Parts are server-side controls that run inside the context of special pages (that is, Web Part Pages) within an ASP.NET application or a Windows SharePoint Services site. They are the "building blocks" of pages in Windows SharePoint Services.
There are now two different Web Part styles in Windows SharePoint Services 3.0. Both are supported, but the ASP.NET 2.0 Web Part is the recommended style for your new projects.
SharePoint-based Web Parts — The older style Web Parts have a dependency on Microsoft.SharePoint.dll and must inherit from the WebPart base class in the Microsoft.SharePoint.WebPartPages namespace. These Web Parts can only be used in SharePoint Web sites. Yet in Windows SharePoint Services 3.0, the Microsoft.SharePoint.dll was changed so that Web Parts written in the older style would be compatible with the Windows SharePoint Services 3.0 runtime.
ASP.NET 2.0 Web Parts — These Web Parts are built on top of the ASP.NET Web Part infrastructure. The newer ASP.NET-style Web Parts have a dependency on System.Web.dll and must inherit from a different base class named WebPart in the System.Web.UI.WebControls.WebParts namespace. These Web Parts can be used in Windows SharePoint Services applications whether Windows SharePoint Services is involved or not, making them highly reusable. If you are creating your Web Part specifically for a SharePoint site, and it will consume the Windows SharePoint Services object model, you can derive from the ASP.NET System.Web.UI.WebControls.WebParts.WebPart base class and add a reference to the SharePoint object model in your project.
The Windows SharePoint Services 3.0 Web Part infrastructure is built on top of a control named SPWebPartManager that is derived from the ASP.NET 2.0 WebPartManager control. The SPWebPartManager control overrides the standard behavior of the WebPartManager control to persist Web Part data inside the Windows SharePoint Services content database instead of in the ASP.NET services database. In most cases, you don not have to worry about dealing directly with the SPWebPartManager control because the one and only required instance is already defined in default.master. When you create a content page that links to default.master, the SPWebPartManager control is already there.
When you create a Web Part Page for a standard ASP.NET 2.0 application, you need to add logic that interacts with the WebPartManager control to manage the Web Part display mode, and generally you also need to explicitly add editor parts and catalog parts to the page along with the HTML layout to accommodate them. Fortunately, you do not have to perform these changes when creating content pages for a Windows SharePoint Services 3.0 site. Instead, you inherit from the WebPartPage class that is defined inside the Microsoft.SharePoint.WebPartPages namespace and it does all the work behind the scenes for you.
Custom Web Parts provide developers with a method to create user interface elements that support both customization and personalization. The term customization implies that changes are seen by all site members. Individual users can further personalize Web Part Pages by adding, reconfiguring, and removing Web Parts. The term personalization implies that these changes will be seen only by the user that made them.