.
Satoshi Nakamoto is the pseudonymous person or group of people who
designed and created the original Bitcoin software,
currently known as Bitcoin-Qt.
His involvement in the original Bitcoin software does not appear to
extend past mid-2010.
There are no records of Nakamoto's identity or identities prior to the
creation of Bitcoin. On his P2P foundation profile, Nakamoto claimed
to be an individual male at the age of 37 and living in Japan, which
was met with great skepticism due to his use of English and his Bitcoin
software not being documented nor labeled in Japanese.
British formatting in his written work implies Nakamoto is of British
origin. However, he also sometimes used American spelling, which may 
indicate that he was intentionally trying (but failed) to mask his writing
style, or that he is more than one person.
The first release of his original Bitcoin software is speculated to be
of a collaborative effort, leading some to claim that Satoshi Nakamoto
was a collective pseudonym for a group of people.
Investigations into the real identity of Satoshi Nakamoto have been
attempted by The New Yorker and Fast Company. The New Yorker arrived at
Michael Clear, a young graduate student in cryptography at Trinity College
in Dublin, who was named the top computer-science undergraduate at
Trinity in 2008. The next year, he was hired by Allied Irish Banks to
improve its currency-trading software, and he co-authored an academic paper on peer-to-peer technology.
Fast Company's investigation brought up circumstantial evidence that
indicated a link between an encryption patent application filed by Neal
King, Vladimir Oksman and Charles Bry on 15 August 2008, and the 
Bitcoin.org domain name which was registered 72 hours later. The patent
application contained networking and encryption technologies similar to
Bitcoin's. After textual analysis, the phrase "...computationally 
impractical to reverse" was found in both the patent application and
bitcoin's whitepaper. All three inventors explicitly denied being 
Satoshi Nakamoto.
Work

Nakamoto has claimed that he has been working on Bitcoin since 2007. In
2008, he published a paper on The Cryptography Mailing List at metzdowd.com describing the Bitcoin digital currency. In 2009, he released
the first Bitcoin software that launched the network and the first units
of the Bitcoin currency.
Version 0.1 was for Windows only and had no command-line interface.
It was compiled using Microsoft Visual Studio. The code was elegant in
some ways and inelegant in others. The code does not appear to have been
written by either a total amateur or a professional programmer; some 
people speculate based on this that Satoshi was an academic with a lot
of theoretical knowledge but not much programming experience.
Version 0.1 was remarkably complete. If Satoshi truly only worked
on it alone for two years, he must have spent a massive amount of
time on the project.
Nakamoto was active in making modifications to the Bitcoin
software and posting technical information on the Bitcoin Forum until
his contact with other Bitcoin developers and the community gradually
began to fade in mid-2010. Until a few months before he left, almost all
modifications to the source code were done by Satoshi -- he accepted 
contributions relatively rarely. Just before he left, he set up Gavin
Andresen as his successor by giving him access to the Bitcoin SourceForge
project and a copy of the alert key.
Motives

Nakamoto's work appears to be politically motivated, as quoted:
"Yes, [we will not find a solution to political problems in cryptography,]
but we can win a major battle in the arms race and gain a new territory of
freedom for several years. Governments are good at cutting off the heads
of a centrally controlled networks like Napster, but pure P2P networks
like Gnutella and Tor seem to be holding their own." - Satoshi Nakamoto
"[Bitcoin is] very attractive to the libertarian viewpoint if we can 
explain it properly. I'm better with code than with words though."
- Satoshi Nakamoto
In the Bitcoin network's transaction database, the original entry has a 
note by Nakamoto that reads as:
"The Times 03/Jan/2009 Chancellor on brink of second bailout for banks"
Some claim this quote implies Nakamoto had great concern or contempt for
the current central banking system.
The smallest unit of the Bitcoin currency (1/100,000,000) has been named 
"satoshi" in collective homage to his founding of Bitcoin.

.