They consist of three cruel, chaotic, dangerous, tricky, power-hungry, arrogant, remorseless, highly sadistic, and incredibly cold and calculating as well as extremely merciless adult witches named Icy, Darcy and Stormy. Icy is mean, petty, maladjusted, vicious, very fiery, and extremely rude. She also has a tendency to let her temper get the better of her when things don't go her way. However, in comparison to her sisters, she is more aggressive than Darcy but calmer than Stormy. Darcy is competent, serious, cool, professional, quiet, reserved, devious, calculating, condescending, very calm, collected, intelligent, and manipulative, and has a more twisted side to her. She seems to be the most level-headed of the trio. Stormy is loud, wild, violent, at times rather childish, and extremely ill-tempered. She is the most aggressive of the trio.
The Trix are the main antagonists in Seasons 1 and 6, the secondary antagonists in Seasons 2, 3 and 5 (as well as Magical Adventure), and return as the Bigger Bads and final antagonists in Season 7. They do not appear physically in season 4.
The Trix are the descendants of the Ancestral Witches who destroyed Bloom's home planet, Domino and created Valtor. It is debatable if the Trix are actual biological sisters or not. But they call each other sisters in some cases.
At the beginning of the first season, the Trix seemed to be more powerful than the Winx Club (with the exception of Bloom), as seen in the episode Spelled where they were able to get the better of the fight, and would have won had Bloom not intervened. For most of the first season, they attended the Cloud Tower School for Witches until they are all expelled after they created the Nightmare Monster in the episode The Nightmare Monster that failed to accomplish its purpose. The Trix stole the Dragonfire and summoned the Army of Darkness/Army of Decay to take over the realm of Magix, but Bloom defeated Icy with her dragon fire while the other Winx defeated Darcy and Stormy.
For the next two seasons they had powerful allies. After being sent to Lightrock Monastery, in the second season it was the infamous Lord Darkar who released them and gave them their Gloomix to increase their power. Darkar double-crossed the Trix in the end before he was killed and they were imprisoned in the Omega Dimension. In the third season, they joined up with Valtor and the three witches competed for his affections, before finally leaving him when he assumed his original, demon-like form and lost all the spells he had stolen when he was fighting Bloom in the 25th episode. Afterward they are sent back to Lightrock Monastery.
They returned again in the second movie, where they sneak into Alfea during the beginning of year festivities, putting a spell on all the food there so that everyone who ate some of it was turned into a toad, and stealing a magical compass which they used to lead the Ancestral Witches to the Tree of Life found in the Pixie Village. The Ancestresses proceeded to absorb all the essence of Good from the Tree, causing all good magic to disappear, and only evil magic to keep on existing. At the end of the movie, the Winx Club, the Specialists, Oritel and Erendor arrive at the haunted city of Havram to find the branch of the Tree of Life which the Ancestresses gave to the king of Eraklyon as part of the deal which forced him to help the Ancestral Witches in exchange for their sparing his kingdom. There, the Trix and their Ancestresses were waiting for them, but the Winx Club girls gained back their powers and Bloom's horse, Peg, became a flying unicorn thanks to the magical branch. Finding the Trix too weak to defeat the Winx Club, the Ancestresses try to possess the Trix, but they attempt to flee as the old witches capture them and possess their bodies. The Trix, now possessed by their ancestors, have become more powerful than ever, as well as mere puppets possessed by the Ancestral Witches. The Winx, the Specialists, Oritel and Erendor take part in the battle, but Erendor is severely wounded trying to save Bloom from Icy-Belladone. Finally, using a Believix convergence, the Winx Club are able to summon the essence of the Great Dragon and extract the Ancestresses from the bodies of the Trix. The Great Dragon's power destroys the Ancestral Witches for good, and the Trix, defeated, fall unconscious to the ground. Helia ties them with a rope and, as the movie ends, they are shown as being suspended from the flying ship the Winx, the Specialists, Oritel and Erendor use to leave Havram, getting angry as Bloom and her fiends fly around them and tease them.