Yu-Gi-Oh! GX (meaning, "Generation neXt" or "Generation X") was a 2004-2008 Anime TV series adaptation of the original 2005 manga (which ended publication in 2011), and formed the first spin off from the original Yu-Gi-Oh! manga/anime.
Set 10 years after the conclusion of the original series (hence "Generation X", meaning "Generation 10") , the series has many similarities with the "Harry Potter" series of books, insofar as most of the action centres around the goings off at "Duel Academy", the most prestigious school for aspiring professional duellists in the Yu-Gi-Oh world.
The series consisted of four complete seasons, although only three have been dubbed into English by 4Kids entertainment, who have stated that there are no plans to dub the final series, which caused some controversy amongst fans due to series 3 ending on an as yet unresolved cliffhanger, which, even though there are numerous fansubbed copies of the 4th series floating about, isn't properly resolved by watching it in the original Japanese, as the westernisation of the series basically made the two versions a little incompatible with each other.
A GX movie was planned, but was ultimately shelved in 2005, with the plot for the movie being developed further and used as the overall plot arc for series 2.
The main character is Jaden Yuki, a 13 year old boy who enrols in duel academy, and who over the course of the series shows that he is at least as talented at Duel Monsters as Yugi Moto and Seto Kaiba. Jaden (as well as most of the main characters as it turns out over the course of the series) has the ability to see "Duel Spirits", a concept introduced in the original series and built upon in this series, becoming a very important plot point from about half way through series 2 onwards.
Secondary characters include:-
Chazz, Jadens main rival and secondary protagonist throughout most of the series, who later comes to be a friend of Jaden (in a similar way to how Yugi and Kaiba held a grudging respect for each other throughout the original series).
Cyrus, Jadens best friend, whose dueling skills are overshadowed by his older brother, but who manages to prove himself time and again (similar in some respects to Joey from the original series).
Dr. Crowler, the head of the "Obelisk Blue" dormitory who takes an instant dislike to Jaden, but whom later is forced to admit he holds a begrudging respect for Jaden and his skills (Dr. Crowler is roughly analogous to "Professor Snape" from Harry Potter, insofar as he is initially an elitist antagonist, but later becomes something of an ally to the protagonists)
Alexis, A female duelist and member of Obelisk Blue, although unlike most members of that Dorm, she is not elitist. She spends most of series 1 searching for her lost bother Atticus, but thereafter is relegated to being a "bit part" tertiary character.
Hassleberry, Introduced at the beginning of series 2, and initially being somewhat of an enemy to Jaden, Hassleberry is an aspiring soldier with an interest in dinosaurs, who ends up being won over by Jadens fun loving attitude.
Aster, Introduced along with Hassleberry in series 2, Aster is a professional duelist who arrives at Duel Academy on a secret mission to take down Jaden (which forms an important part of series 2s story arc), however he discovers that he has been manipulated the two become firm friends and team up to fight that series' bad guy.
Initially, the series is played more for laughs than anything else, and includes numerous instances of characters "breaking the fourth wall" and making various pop culture references (films referenced include "Ferris Beullers Day Off", "Return of The Living Dead", "Hellraiser", "Beverley Hills Cop" and many more) , however as the series goes on, the laughs become fewer as the main storylines become much darker and more "anime" in tone, but the series still retains much of its humourous content.
In addition to this, the series generally features one main story arc, divided into two chapters per season, with the first chapter acting as a "set up" for the finale.
Series 1 - Jaden at Duel Academy/The Sacred Beasts.
Jaden settles in at Duel Academy, making friends and enemies along the way. Then a mysterious group of duelists known as "The Shadow Riders" arrive and Jadens group of friends are called upon to stop them from aquiring the three "Sacred Beast"cards, which are basically evil versions of the "Egyptian God" cards used in the first series.
Series 2 - The GX Tournament/The Society of Light
Duel Academy is chosen to be the site of the GX tournament, a tournament created to celebrate the tenth aniversary of the infamous "Battle City" tournament hosted by the Kaiba corporation. During the course of the tournament, a clairvoyant duelist known as "Sartorius" begins converting people to his pseudo religion, "The Society of Light", and Jaden, along with his friends and former enemy Aster Phoenix, must stop him from taking over the world.
Series 3 - Hunt for the Rainbow Dragon/The Supreme King
Duel academy finds itself being hurled into one of the alternate dimensions where Duel Spirits are real creatures, and only the power of the mythical Rainbow Dragon card can get them back. Later, Jaden and his friends return to this dimension in search of their missing friend Jesse, and find themselves having to tangle with the forces of the supreme king, as well as the ambitions of Adrian Gecko, a power hungry duelist who wants to be king of his own world. Jaden also finds himself up against an old foe from his childhood who he thought had gone forever.
Series 4 - Old enemies/Graduation (This series has never been dubbed into English)
Jaden returns from the alternate dimension victorious, but as a changed man. Over the course of the series he meets and defeats many of his old enemies and finally graduates from Duel Academy, but not before he faces a duel with destiny...against Yugi himself!.
I personally enjoyed Yu-Gi-Oh GX, initially I thought I wouldn't like it due to hearing bad things on the grape vine, but, the humourous content, combined with the fast paced and more coherent storylines (compared with the original series' first season) and interesting characters won me over in the end.