It was an important battle. At the Second Battle of Ypres, the gas attack was effective, and that's why there are gas bombs today. =)
During WW1 there were 3 main battles fought at Ypres. It was a strategic strong point known as the Yrpes Salient. This means that it stuck out into the German's front line. This was a highly important position to defend for the British/Allied forces and one that the Germans wanted. In the first battle of Ypres, the British captured the town and reoccupied the lost ground. In the 2nd battle from April 22 to May 25, 1915, the Germans made a great attack using gas for the first time in an attempt to retake Ypres. The final battle of Ypres, better known as Passchendale, took place between the 21st of July and the 6th of November, 1917. The 36th Ulstur Division lost many men here and fought with outstanding bravery along with all commonwealth troops. The Meningate stands as a mounument to all the soldiers who died in battle.
I may be wrong, but I'm pretty sure Passchendaele was just near Ypres. And so is Wikipedia :P Also I just spellchecked that whole paragraph above.