Could Silverlight and WPF be interchangeable?
I mean, using Silverlight for Windows applications and WPF for web applications.
Silverlight and WPF are similar, but not interchangeable.
Silverlight and WPF are two different technologies intended to solve different problems. Silverlight has out-of-browser functionality, but must be delivered over the web in a browser first. WPF has XBAP, but it's still a WPF application. They are both XAML-based, so there is some limited ability to share resources.
If you are designing an application, select the technology that is most appropriate to the problem and the target audience. Keep in mind that while Silverlight runtime installs are growing fast, they still lag far behind others. Some people flatly refuse to install it. WPF apps can be distributed like desktop applications with an executable.
Update: In my experience, WPF XBAP applications don't behave well in browsers that aren't IE.