VS 2010 has shipped with wide variety of project types for asp.net/sharepoint developers, in this post i will clarify the use for the most common project types and items used for building sharepoint 2010 solutions:
1) Import Sharepoint solution package: this project template allows you to import existing wsp files that you developed before.
2) Import Reusable workflows: this template allows you to extend created/developed workflows using SPD in VS 2010. VS 2010 workflows provide more capabilities that doesn't exist in the SPD workflow designer. Once you are done, you can also deploy using VS 2010.
Note: once you modify the imported workflows from SPD you wont be able to open it back using SPD.
3) State Machine workflow: in this template you can create state machine workflows using VS 2010.
4) Application page: Use this page to add any asp.net pages in sharepoint sites.
5) User Control: this is a regular ascx user control that you can drag-drop it on your application pages.
6) Web part: this is a normal webpart that we use to develop since MOSS 2007.
7) Visual Web part: This is a visual webpart that you dont need to render it is interface, it contains a webpart and a usercontrol item.