I want to watch Fate/Apocrypha but I didn't watch Unlimited Blade Works. So, is Fate/Apocrypha the sequel to Unlimited Blade Works?
According to the MAL page, Fate/Apocrypha is set in a parallel world to the Fate/stay night series.
(First sentence of the synopsis):
The setting is a parallel world to Fate/stay night where the Greater Grail mysteriously disappeared from Fuyuki after the Third Holy Grail War.
You only need to have watched the original series, in this case. Apocrypha is not particularly important to the series.
(Also see Senshin's answer for the full order)