Izumo was the lead ship of a class of WWI-era armored cruisers, built in Britain for the IJN, and closely related to the Asama-class. She served as a flagship in many battles, and later as a training ship in WWII until she was sunk by a US air attack during the end stages of the war. Her wreck off Kure was floated and broken up for scrap. Historically, Izumo was armed with four 20.3cm guns in twin turrets, and multiple secondary guns in casemates around the ship. She was also equipped with four torpedo tubes and a Nakajima E4N seaplane (lowered to the sea by a crane from where it took off by its own power), though these are not featured in game.
After the excellent Asama, Izumo feels like a downgrade which it is. For one, its secondary batteries can no longer fire in virtually any direction in full number, instead you can only fire two guns directly ahead or behind you. Given the lumbering nature of the ship this makes being chased quite dangerous, so watch out for approaching DDs. Secondly, the ship's upgrades are largely useless, and it's not a bad idea to ignore them and head straight for the next ship. Aside from this, Izumo plays exactly the same way as Asama. Stick close to battleships, lay down suppressing fire on smaller ships, and harass this big ones while they're distracted. But remember, you are slow, and cannot defend against air attacks! Take the time to learn the cautious opportunism that will be the heart of CA gameplay all the way through.