Just a few info instead of a crash-course:
Croatia is very close to Greece and had colonies in the ancient times. It also has a sea border with Italy and huge portions were under ancient Rome.
This left many very old varieties. Some can't be traced anymore and some gave some natural clones or crossed with others to create some very new and unique grapes.
Croatia has over 120 known indigenous varieties.
Grk may win by a hair, but some come very close:
Pošip, Maraština, Kurtelaška, Škrlet, Bogdanuša just to name a few whites.
Babić, Lasina, Ninčuša, Crljenak, Plavac, Teran to name a few reds.
Not all of them are being produced today. Many are coming back. And there are also those that made a new home here like Pušipel which is in fact Furmint (of Tokaji fame), making some gorgeous dry whites.
By consumption, number 1 is Graševina (Welsh Riesling which, as it seems, originates from around Ilok in Slavonia).
Number 2 is Istrian Malvasia, a specific clone of Malvasia that thrives only in Croatia's penisula of Istria
Number 1 red is Plavac mali, a part of the Zin family (I guess you've heard by now that Zin originates from our shores), it's very, very tannic. Very tannic! I'm talking xinomavro or nebbiolo level of tannin.