The launch of the British ship-of-the-line HMS Dreadnought, in 1906, was an event comparable to the first detonation of an atomic bomb in 1945. It completely revolutionised warfare and rendered obsolete most if not all capital warships of the time. "Dreadnought" quickly became the type name for all ships of her kind; all older capital ships were soon to be called "pre-Dreadnoughts".
Before World War I, ships-of-the-line were what ICBMs are today: the biggest and meanest weapon system of all. Their numbers were carefully regulated by written and/or unwritten treaties. Building new ones could upset the international balance of powers. By building the Dreadnought, the British navy essentially forced all navies in the world to upgrade their fleets (this was much like an MS Office release). The German Empire realised that in the ensuing arms race, they stood a chance of overtaking Britain and, for the first time, becoming the leading naval power in the world. This German decision ultimately caused World War I.
Now, what was so special about the Dreadnought? It was the first "all big gun battleship". Unlike ships-of-the-line before, which had a mix of many cannon of different calibers mounted in turrets all over the ship, with usually only four guns (the "main battery") being of the heaviest caliber, the Dreadnought had a main battery of ten 305 mm (12 inch) guns. It was also turbine-powered, which meant higher speed (21 knots, i.e. three knots more than usual) and less vibration.
The years between 1906 and 1914 would see a frenzy of Dreadnought construction, ultimately leading to mile-long formations of Dreadnoughts in keel line affronting each other in the few larger sea battles of WWI.