- In this case, is the Arduino a Client or Server?
A client makes GET requests. A server responds to GET requests. Which is the Arduino to be doing?
It looks to me like it needs to be a server, so clients (like a web browser on the PC or the phone) can make requests of it.
- If Arduino is a server, is it possible for it not to host any webpages and it will just process the string command send throught its IP address. e.g. htt://192.168.1.177?Z1L1ON
Yes, but wouldn't it be better to serve up a small page - just a message, really, like "Success" or "What were you thinking?"?
- In this case how can the Android talk to my Arduino throught the IP address?
The Arduino will have an IP address. Enter http://[:port} in the browser, and a GET request is made.
If the Android supports making direct GET requests, no browser is needed. But, if it supports running a browser, like an iPhone does, then there is no reason to program the Android at all.