"Netdecking" is the practice of copying a Championship-winning Deck as used in either Nationals, Shonen Jump Championships, World Championships, or any other high-profile event where Deck Lists are posted.
Netdecking is often referred to negatively, being called "a practice that involves zero creativity and is usually done by players who are only interested in a cheap, effortless win, believing they can only achieve such a victory by copying an established winning deck" (see Cookie Cutter). For this reason, it is a practice that is considered to be shunned upon by many players. Some Duelists argue that while the decks that win major tournaments are indeed relatively the same, it is because the decks are of a tried and true method, and have shown themselves to be consistently placing high, so people who want to win will try and have the best odds they can have, as it is human nature to be competitive and wanting to be better than other people. It is also debated that there is no reason for a person who uses an Original Deck to be seen as higher than a netdecker, as it is how the individual enjoys playing that counts. Some duelists also blame the formats created by Konami for allowing that to happen.
Although similar, the terms "Netdecking" and "Cookie Cutter" are subtly different. Whilst "Cookie Cutter" refers to a deck without a distinct strategy (simply a deck of the best and most popular cards available at the time), netdecking refers to creating a deck by copying the deck of another.
Yu-Gi-Oh! GX - Episode 018 and Yu-Gi-Oh! GX - Episode 019 took a jab at this phenomenon with Dimitri, a character who imitated the decks of other duelists to the point that he imitated the duelists themselves as a result.