TCEC is the unofficial world championship in the same sense that the Tour de France is the unofficial world championship cycling. So not really, but it is just more popular and considered harder.
There are many differences between TCEC and WCCC. The most notable are that engines are not allowed to use their own books in TCEC, and all have to run on the same hardware. So TCEC is not really a Chess championship, but a strength test for middle-game software. It can be compared to a thematic tourney for humans, where all grand masters have to start playing from a position well beyond anyone's opening repertoire, unknown in advance (so they cannot prepare), and where they would suffer a reduction in thinking time proportional to the size of their head. Furthermore, TCEC is an invitation tourney; participation is not open to all, but the organizer decides who is going to play.
The WCCC is about getting the best Chess performance no matter how you do it. If you want to build your own hardware, like Deep Blue once had, or want to play on a distributed cluster of 100,000 PCs, it is all allowed. The games start from the standard FIDE positions, and participants use their own, prepared openings. Participation is open to all programs, under the restriction that every program (or program part, such as an opening book) can only participate in a single copy, and that the author backs its participation. (I.e. you cannot just buy Komodo and then have a go at the world title, playing against 20 others that did the same or downloaded Stockfish.)
There are also some differences in how they are organized: WCCC is an 'over the board' tournament, which requires physicalpresence on site. As a consequence it only has a limited number of rounds, and (barring playoffs for tie breaking) participants would only play each other once. TCEC is what is known in computer Chess as a 'basement tourney': one guy in a room with a computer installs many different engines on it, and has those playing against each other one after the other, using that same computer equipment in turn. As computers are patient, and letting them run fully automated is cheap, there can be many games between the same participant, reducing the influence of luck, and leading to a fully predictable outcome even between opponents that for all intents and purposes should be considered equally strong.
What is the real difference between the WCCC(World Computer Chess Championship) by the International Computer Games Association and TCEC(Thoresen Chess Engines Competition)?
I have recently read that the TCEC is considered unofficial world computer chess championship and the main question is if anyone knows why there is an official and an unofficial tournament.