I am an android application developer (trying to be one) and I want to learn how to communicate with web sites from my applications. However, I don't know where to start. Can you point me in the right direction?
PS : I believe "web communcation standarts" is wrong term but i don't know how to call them.
I'd suggest that you learn by doing. Pick an idea - let's say you want to upload a picture from your Android phone to facebook.
Now go and research how to do that. Facebook exposes what they call the "Graph API" for all fb resources, including users, statuses, and photos, among other things. The Graph API is a REST API - in other words each item on the backend is modelled as a "resource" which is accessible from a URL. (Learn more about REST on wikipedia). Send a GET HTTP message to the given URL and you can get information on the resource. Send a POST to the container of that resource, and you can add another of whatever it is. In this model, to add a photo, you POST to the album URL. Read on FB for how to do that. They call it "publishing".
But reading that doc page you learn that in order to POST you need an authorization token. Your app needs publish_stream permissions. How do you get that? Keep reading in the FB doc and you'll see a link to the Authentication and authorization guide. That tells you about how to use OAuth 2.0 in Facebook apps.
Your app will send its first feeble message out, and get an error response. Why? You'll want to examine the message. so you google and figure out that you can use an http debugging proxy to help you, something like Fiddler2 for Windows, or Charles for other systems. So you download that, set the proxy on your Android device, and now you can view outgoing HTTP messages and their responses on your PC. Keep plugging along, learning what you need to accomplish the task in front of you: in this example, posting a photo from an android device to Facebook.
After you reach that goal, you will have learned quite a bit of practical knowledge for this domain of problem. Then you can try a new goal, and work on that.
After a few iterations of this, you will have learned how to learn, which is even better than learning REST or OAuth. If you know how to learn, the sky's the limnit.