Based on the original Manga by Kazuki Takahashi, Yu-Gi-Oh! was a 2000 - 2005 anime which consisted of 5 complete series, 1 sub series and 1 movie.
The series is a fairly faithful translation of the original manga, however obviously some changes had to be made due to violent, sexual and religious content, changes which also included new characters and story arcs being created to replace unfilmable ones from the manga series.
The series focuses on Yugi Moto, a young man who has a particular knack for puzzles and gaming.
at some point prior to the series' beginning, he manages to solve a puzzle given to him by his grandfather, who in turn found it during his time at an archaeological dig in Egypt.
As it turns out, the puzzle is one of the millennium items, a series of seven magical artifacts created 5000 years ago by the nameless pharaoh to trap an ancient and dark magic which very nearly destroyed the world.
Although he doesn't realise it at first, Yugi's completed puzzle contains the spirit of the nameless pharaoh, in whom he finds a kindred spirit, as both of them have a skill for gaming and puzzles, and Yugi can call on the pharaohs skill during his favourite game, "Duel Monsters".
The series itself consists of five story arcs, all of them revolving around Yugi and his friends battling against evil forces who use dark magic in an effort to conquer or destroy the world by various means, and these schemes are usually thwarted by the playing out of various games which have some sort of connection to the ancient world.
To people who are familiar with the card game, the first series may seem strange as the depiction of gameplay on screen is not the same as how the game is played in real life, this is due to the 1st series using the games original Japanese rules, which bore more resemblance to the "Top trumps" card game, albeit with a slightly altered game mechanics to make it more "tactical". This discontinuity was addressed during the second series (aka - the "Battle city" story arc) when the "Battle City" rules were used in a competition organised by Seto Kaiba, which were pretty much identical to the rules of play for the real life CCG, with slight discrepancies to allow for better on screen action (for example, certain cards and/or their in game effects were misrepresented, and cards were used that still don't exist to this day as plot devices)
As the series went on, the story lines became more fantastical, and eventually Yugi and the Pharaoh learned the truth that the Pharaoh had locked away within his own mind, which drew the series to its final conclusion.
All in all, the series was very good, albeit being a bit of a niche interest one. I first discovered the series being shown on Sky one on a saturday mornings and found it to be quite interesting when I gave it a chance, despite having no interest in it prior to this. I liked the way how the characters developed over time to become more than the simple archetypes they were initially presented as, and it also helped generate my interest in the real life card game and video game series.
Yu-Gi-Oh! has since gone on to have spin off series' in the forms of Yu-Gi-Oh! GX, Yu-Gi-Oh! 5Ds and currently Yu-Gi-Oh! Zexal, all of which have their own plot lines but all of which have their core focus on using the card game as a plot point, so its obvious that the series is stil as popular now and will continue to be popular for some time to come.