Most developers who are moving from other web development environments to Erlang and Yaws will have used other web servers such as Nginx or Apache. The Erlang Yaws web server performs the same basic tasks, but the details of performing common actions are often different.
Erlang is not only a language, but also a runtime system and something that looks a lot like an application server. As such, Erlang and Yaws (or other web servers) will fill the same role as Apache/PHP/MySQL and other components all in one system.
The major differences between Erlang/Yaws and Apache/PHP have a lot to do with how Erlang tends to set things up. Erlang assumes that systems will be clustered, and processes in Erlang are somewhat different from those used in many other systems.
If you’ve used Apache with mod_php, you may remember that each request is handled by a process or thread (depending on how things are set up). The classic Common Gateway Interface (CGI) would start a new process for every request. These threads and processes are constructions of the OS and are relatively heavyweight objects. In Erlang the processes are owned not by the OS, but by the language runtime.
When building an application with Apache and PHP, for each request the web server must bring up a copy of the PHP interpreter and quite possibly recompile the various bits of PHP code that are to be run. This is an expensive operation. By comparison, in Yaws the Erlang code is probably already compiled ...