Difference between Fate/Stay Night and Fate/Stay Night UBW? I'm quite confused and I'd like to know what's the difference between Fate/Stay Night and Fate/Stay Night Unlimited Blade Works?
As mentioned by senshin in his answer to the question What is the viewing order of Fate-related works?


Fate/stay night is a visual novel originally released in 2004. The story has three routes: Fate, Unlimited Blade Works, and Heaven's Feel.
The first TV anime series by DEEN in 2006, titled Fate/stay night, mostly adapted the Fate route.


   However, Caster's plan to prematurely summon the Grail was taken from the Unlimited Blade Works route, and Sakura and Rin's fight was taken from the Heaven's Feel route

The movie by DEEN in 2010, titled Fate/stay night: Unlimited Blade Works, adapted a highly condensed form of the Unlimited Blade Works route.
The second TV anime series by Ufotable in Fall 2014 and Spring 2015, also titled Fate/stay night: Unlimited Blade Works, is another adaptation of the Unlimited Blade Works route.
There are plans for a movie/series of movies by Ufotable which adapts Heaven's Feel route.