Web server + programming languaje
Communication method (JSON, SOAP, plain HTTP requests)
Some UI thing
For example, you could build the next great Web 2.0 app with:
- dbf files, Clipper running as CGI on top of IIS, JSON and Flash UI. Just joking.
- PHP+JSON+HTML+Prototype (like I describe in this article in spanish, titled "Client-Server Arquitecture with AJAX and JSON)
This ended up being a buzzword salad...