Satoshi Nakamoto is the (to this date) unknown person or group who invented bitcoin. He authored the bitcoin whitepaper and was active in the development up until December 2010. His idea of a decentralized digital peer-to-peer currency was revolutionary and never achieved before.
In October 2008 Satoshi Nakamoto published the first whitepaper under the title „Bitcoin: A Peer-to-Peer Electronic Cash System“. A few months later he released the first Bitcoin software that launched the network. You can read the whitepaper here: bitcoin.org/en/bitcoin-paper
Around June 2010 Satoshi Nakamoto handed over the control of the codebase and some related domains to the software developer Gavin Andresen. Satoshi was then never heard again.
Since all transactions of the bitcoin network are logged and are accessible for everyone, it is easy to trace the first transactions associated with Satoshi Nakamoto. His known address contains roughly one million bitcoins but they remained untouched ever since. If he would sell his stack, he would instantly become one of the 100 richest persons in the world. And probably crash the whole crypto market, haha!
Although his identity is unknown, there are some traces he left behind. His English language was perfect and contained some British spelling (e.g. „bloody hard“). So it is assumed that Satoshi Nakamoto (or at least one person of them) has roots in Great Britain. This is strengthened by the fact that the genesis block (the first block ever created) contained a message („The Times 03/Jan/2009 Chancellor on brink of the second bailout for banks“), which is linked to a news headline of the London’s The Times newspaper that day.
The community also analyzed the time stamps of all his bitcoin forum posts (>500). And since there haven’t been any posts between 5 am and 11 am GMT, it was supposed that he wasn’t at least living in Japan (like the name would suggest).
One of the earliest bitcoin developers was Laszlo Hanyecz. Yes, it’s the guy who purchased pizza for 10,000 BTC. And since he was working on the bitcoin code together with Satoshi Nakamoto, they exchanged hundreds of emails (although they never met in person). He described the communication as kind of weird because Satoshi Nakamoto always dodged all personal questions. They were simply left unanswered, up to the point were Satoshi Nakamoto appeared paranoid. Laszlo concludes their collaboration saying "People like to identify with heroes or villains, but in the cryptosphere, your code has to speak for itself. Charisma and being an interesting person only gets you so far when you're a developer. Ultimately, you'll be judged on the quality of your code and your idea."
A lot of people have been alleged to be the person behind Satoshi Nakamoto. For example Hal Finney, Nick Szabo, Elon Musk or Craig Wright (haha!). But since this is all 100% speculation, let’s not waste time here.
Another rumor says that the NSA used stylometry to identify him. In a nutshell: Over the years Satoshi wrote thousands of posts & emails, the NSA identified his 50 most common words and analyzed all his text to find the frequency of those 50 words. This (together with some advanced math) resulted in a fingerprint of his language. The NSA then compared this fingerprint to all of their PRISM & MUSCULAR data (aka the whole internet). It took them less than a month and resulted in a positive match! Of course, the identity wasn’t revealed, but the Obama administration wanted to make sure that no Russian or Chinese spy was behind all of this. They had the fear that bitcoin might have been used as a potential future weapon against them.