Mtgox was one of the first online exchanges, where you could exchange fiat currencies for bitcoin and also the other way around. It was founded by Jed McCaleb in 2006, the name was based on a website where users could trade game cards as stocks named Magic: The Gathering Online Exchange. Mt.Gox was sold to Mark Karpeles in the year 2011. Three years later, in 2014, Mt.Gox was closed after a hack that resulted in 850 000 Bitcoins being stolen/lost.
The process of compensating the users for their loses is still ongoing, as not all Bitcoins on the exchange were lost ... due to the BTC value going up a lot since, there is now more funds (fiat) available than the total loss at the time of the hack.