Dear Mostafa,
ET200s Is a distributed I/O ,means it's installed in the field to collect data from sensors or actuate actuators .These sensors can be analog digital or mix .then it transfers these data through profibus .and the ET200S of Siemens is the compact type, what means that it can't be installed on rack Like ET200M which is a main communication unit and signal modules can be installed through rack .this rack can be extended and so on... . and ET200s comes with its I/O already exist.
Hope this will simplify it.
Yours
Abdelaal