As mentioned already the distinction between the two is becoming increasingly blurred, as web based applications become more and more sophisticated.
To answer your questions:
1. This is like asking how long is a piece of string. How much you make will depend on what you are building, and who you are building it for.
2. Some languages like C# and Java are used to develop both web applications and desktop applications.
3. Something like PHP should be fairly easy to pick up compare to say C++, but bare in mind what I said earlier, some languages can be used for both. It is difficult to generalise really.