Gallipoli is significant to australians because it was the first time that the australians really distinguished themselves (in combat) as their own country and not just the old prison state of england. It was a proud and significant moment just as D-Day had been for americans, but with even more at stake.
Gallipoli was of huge tactical importance because if the Allies were able to capture Gallipoli and the Dardenelles, they could take Constantinople and open another front against the Germans. At this time in WWII there was a stalemate on the western front in europe and opening another front against the Germans would be a huge advantage for the allies.
The Turks had been humiliated in WWI and just about all of their previous Ottoman empire was gone. They were an ill-equipped army, and were very weak.
At the time a great military leader arose, named Mustafa Kemal (later Ataturk) who successful defeated the allies at Galipoli and literally drove them back into the sea.
The Turks were not nearly as well armed or supplied, but they had the tactical advantage of the higher ground in the battle and the Allies had to scale steep, sharp cliffs. They also had the great leadership of Ataturk and the fierce determination to defend the last bit of their homeland.
The battle turned into fierce trench warfare that lasted months and claimed hundreds of thousands of lives. The movie Gallipoli is about the trench warfare specifically and how brutal it could be.
When the Allies finally were driven from Turkey, Mustafa Kemal rode this momentum to be the leader of all of Turkey. He remade Turkey into what it is today and is a God there. His picture graces every monetary note and coin, restaurant and hotel, and likely every home. He was a great military leader, diplomat, and had incredible charisma. Some of the things he said in his speeches were so powerful and moving. Kemal, who later changed his name to Ataturk, literally, "father of the Turks", is everything to Turks and is not to be taken lightly. An important thing to remember as you travel through Turkey.