A class of capital ship of the dreadnought era, originally the brainchild of Jackie Fisher, who was responsible for bringing in the all-big-gun, turbine engined battleship. Battlecruisers were intended as screening and reconnaissance units for the main battle fleet, sacrificing either armour (the British model) or gunpower (the German style) for increased speed, usually 6 or 7 knots faster than the battleships of the era. As such they were also able to chase down and destroy armoured cruisers who they could match for speed and outgun; this made them a deterrent against commerce raiders and other independent cruiser units such as those operating in colonial waters.
Prior to the First World War, for all intents and purposes only the British and the Germans built battlecruisers. These were vessels of around 20 000 tons displacement, armed with eight to ten 12" or 13.5" guns (British) or 280 mm (German) guns. During the war, the battlecruiser squadrons of the Royal Navy's Home Fleet and the effectively blockaded German High Seas Fleet skirmished on several occasions during sorties by the Germans into the North Sea and to bombard British coastal towns: the battles of the Dogger Bank and Heligoland Bight were among these actions; two British battlecruisers were also responsible for the distruction of Admiral von Spee's Pacific squadron at the Battle of the Falkland Islands in 1914.
The battecruiser squadrons screening the main fleets were among the most heavily engaged units at the inconclusive Battle of Jutland in 1916, the war's largest naval battle. The under-armoured British ships came off decidedly the worse when confronted by the main German line of battle, with three vessels - Indefatigable, Invincible and Queen Mary - sunk with massive loss of life due to magazine explosions ("There's something wrong with out bloody ships today" remarked Admiral Beatty, but the problems were probably rather more deep rooted; recent marine archaological investigations strongly suggest that insecure ammunition handling practices - the battlecruiser squadron was very competitive about firing rate - were probably to blame) while the Germans lost one, the Lützow.
By the end of the war, the speed of newly built battleships had matched that of the original battlecruisers, so the newer models had to be still faster to be of any use at all. The British built three rather extreme vessels with almost no armour but very low draughts for shallow water operations - the Furious, Courageous and Glorious - with an eye to a rather far-fetched plan for an operation in the Baltic which was fortunately never implemented; they were all eventually converted to aircraft carriers. The same fate came to the US Navy's planned inter-war battlecruisers, the Lexington and Saratoga. The Japanese built the Kongo class at this time but their armour was upgraded and they were basically of battleship standard; none of the other leading naval powers of the time - France, Italy, Austria or Russia/the USSR - saw fit to build any ships in this class.
During the period of interwar disarmament under the Washington and London naval treaties the Royal Navy kept only three battlecruisers built in the closing years of World War I until the outbreak of World War II, the Hood, Repulse and Renown; only the latter was to survive the war, the Hood - the pride of the Royal Navy - falling victim to plunging fire on its encounter with the Bismarck and blowing up just like its earlier counterparts at Jutland (an Admiral Hood, one of the eponymous naval family, was in fact killed on the Invincible) and the Repulse sunk with the battleship Prince of Wales by Japanese air attack off Malaya during the advance on Singapore in 1941.
The French Navy built two 23 500 ton ships in the 1930s, Strasbourg and Dunkerque, intended to counter the German pocket battleships (which were essentially designed as fast commerce raiders).
The Germans, rebuilding a fleet from scratch under the restrictions of the Versailles treaty and the later the naval treaties, built two vessels, the Scharnhorst and Gneisenau (the same names as the main ships of Von Spee's WW1 Pacific squadron, victorious at Coronel and defeated at the Falkland Islands), which were de facto battlecruisers after the German tradition - fast 35 000 ton ships with a moderate 9 x 280 mm gun armament, strong enough to overpower anything British they could not outrun (except the Hood). They were among the most active of German surface vessels during the war, although that is a fairly relative statement. The Scharnhorst was sunk at the battle of the Barents Sea in 1943, while the Gneisenau survived until bombed in dry dock at the close of the war.
The only class of battlecruisers actually builtduring the Second World War were the three of the American Alaska class (others were Guam and Hawaii), officially classed as "large cruisers" but along the same lines as the Scharnhorst in terms of size; they were designed as a result of an intelligence SNAFU, to counter a supposed new Japanese design like the German pocket battleships which never actually existed; they saw limited service with carrier strike groups in the last year of the war.