What was the first airplane fitted with a hydraulic system?
What was the first airplane fitted with hydraulic system?/Where was it used?


If we exclude hydraulic disk brake systems. 

The Lockheed Constellation most likely had the first hydraulic flight controls (introduced in '43) 

Introduced in '27 the Fairchild FC-1/FC-2 had hydraulically operated landing gear and disk brakes. 


  What are the fundamental differences between the first systems and
  those that are used now?


Depending on how you look at it, little. Hydraulics are hydraulics. The systems have gotten more powerful but that may be related to the available power to drive the pumps. We arguably have gotten better at building them and they are perhaps more reliable but they are still more or less the same in operation.