The most unique feature of seaside is its approach to session management: Seaside models an entire user session as a continuous piece of code, with natural, linear control flow. Pages can call and return to each other like subroutines. Complex sequences of forms can be managed from a single method. Objects are passed by reference rather than marshalled into URLs or hidden fields. Seaside fully supports the backtracking and parallelism inherent to the web browser.
So unlike in the Java servlet model seaside does not need a separate handler for each page or request.
Other features
a callback-based event model
a "transaction" system for auto-expiring pages
programmer-friendly HTML generation
a system of reusable and embeddable UI components
web-based development tools. If an error occurs you may fix it by using the web browser and resume the execution.
When you install seaside you are asked for an administrator user acount name and password.
Use
http://localhost:9090/seaside/config
to access an application which lists the preinstalled applications and allows you to add your own.
The most elaborate example included is a Sushi store application.
http://localhost:9090/seaside/store