In a SharePoint environment, writing your own code usually means writing Web Parts. Web Parts amalgamate easily with the rest of your SharePoint site, leveraging its existing appearance, organization, security, and data management. What’s more, Web Parts are reusable. You can write them once and use them in as many pages as you like.
Consequently, building Web Parts is one of the most common tasks you will undertake as a developer. SharePoint 2010 and Visual Studio 2010 enable you to create Web Parts quickly and easily.
Web Parts work differently than ordinary Microsoft ASP.NET pages do, and much more differently than legacy ASP pages. Thus, developing and deploying a Web Part requires a different approach and a somewhat different tool set.