Actions are the primary purpose of rewriting. The in-built actions include rewrite, redirect, not-found, forbidden, etc. You can set cookies, add headers, and set properties. You can also create your own actions if the in-built ones are not enough.
Conditions control which actions get executed. You can use in-line conditions (on the action itself), or you can use the special if conditional action to group a series of related actions.
Expressions are used in various places to generate a replacement value. Rewrite rules can use expressions that include references to matched expressions, properties, headers, mappings and transforms.
Transforms provide a transformation on a text input to a different output. The in-built transforms include lower to lower-case the text, upper to upper-case the text, etc. You can define static mappings or create a custom transform.
When you start implementing your own actions and implementing your own conditions, you will need to create parsers which will parse the action and condition from the configuration file.
Error Handlers
You can specify error handlers which will get invoked when a specific error occurs (e.g., 404 File Not Found).
If you want to know more about the operations of UrlRewriter.NET, you can add a logger or even write your own custom logger.
API Documentation
The UrlRewriter.NET API documentation provides complete documentation of the classes you can interface with if you want to extend the UrlRewriter.NET component.