An alliance is when two or more contestants join forces in order to vote someone off, or get farther in the competition with teamwork. Throughout the show, many alliances have been formed, and few have succeeded.
The alliance was formed by Courtney as to take control and assert her dominance as the leader on the Screaming Gophers team. She recruited Dakota and Felicity off the bat, but lost control of the alliance when Courtney switched to the Killer Bass team. Dakota strayed, but Felicity stayed loyal when they reunited at the merge. Alejandro hijacked the alliance with Duncan and made it his own, where they proceeded to scheme and antagonize the rest of the competition until Noah revealed them, and they split apart when Alejandro decided to betray Felicity to elimination in That's Off The Chain.
Heather desperately wanted an alliance within the Screaming Gophers team. She recruited Beth easily, but stole Abigail's diary and threatened to read it aloud the way she did with Gwen's if she didn't join her. Heather easily recruited Owen as well, and tried to get Vanessa to join, but when Vanessa tried playing hard to get, Heather cut her loose and had her eliminated in Phobia Factor. Noah recognized Abigail's misery and stole back her diary and got rid of it in If You Can't Take The Heat..., dissolving Abigail from the alliance, and making Heather lose ground. Heather was eliminated that night, ending the alliance for good.
Four alliances were formed in Total Drama Action Do Over, but only two were formed by contestants themselves. Chef and Chris each made an illegal alliance of the season to spice up the drama and try and get a split of the money utilizing Shin and Brick respectively. For the contestants, Vanessa formed her own alliance before the kidnapping occurred where Vanessa was replaced with Heather, where she proceeded to take control of where Vanessa left off. Justin attempted to make an alliance too, but was short lived.
Vanessa formed an alliance VERY early with Scott and proceeded to help each other sabotage the Killer Grips, Scott's team. When Scott got blackmailed by Dawn, he had to quit. Vanessa instantly recruited Anne Maria and later Izzy. When Harold joined, Vanessa became less malicious due to his understanding nature, but in Dial M For Merger, Vanessa was kidnapped and secretly replaced with a disguised Heather, who took over the remains of the alliance. Bridgette persuaded Anne Maria to quit when "Vanessa" was becoming too antagonizing. In Love At First Fight, it's revealed that it was Heather since Dial M For Merger. Heather attempts to trick Eva into being in an alliance, which works until Dawn intervenes, but manages to convince Dawn to eliminate Eva anyway in a one-instant alliance offer. Harold joins Heather in a fake "mutual agreement" deal to lower her guard, but fails. The string of alliances stop at Top Dog.
Chef formed an illegal alliance with Shin to help Shin "man up" and to get a split of the million, seeing Shin as a potential candidate for the win, but in the end, Shin's will was destroyed when Bridgette discovered the alliance and broke up with him. Shin quit the alliance once Bridgette did so, since Shin wanted to include her in it. Once the breakup occurred, there was nothing else keeping the alliance bound.
Chris wanted to counter Chef's illegal alliance with Shin by forming one with Brick by bringing him back to the game, and threatening to keep Brick out of his favorite boot camp if he didn't agree. Brick eventually let his good will win him over and quit the alliance along with being forced to quit the game as well, per the producer's demand.
Justin attempted to make a Guy's Alliance after his failed attempt to get one with Katie. He waited until after the merge to try and form it, and succeeded with Harold and Mike, although being rejected by Brick. Justin attempted to get Eva to join using his flirtation skills, and it worked for a short bit. When he used the same skills unsuccessfully with Dawn in Love At First Fight, Eva was enraged, and Justin was ostracized and lost complete control of the alliance after that.
There were a total of four alliances in this season. The founders were Alejandro, Scott, Jo, and Mel. Scott's alliance was overtaken by Courtney once he let them down, and Alejandro, on his own, destroyed Jo's alliance, but immediately afterwards was destroyed himself. Scott/Courtney's alliance is the most successful alliance to date, getting three solid members to the final three.
Jo made the first alliance of the season, which infamously did little to nothing due to the 'incompetence' of her alliance mates, Lindsay and Cameron. Lindsay comically forgot about the alliance's existence on a daily basis. Once Alejandro returned, Jo finally had a cause for her alliance, and they uncovered a lie created to entrap Nathaniel to Alejandro. Alejandro displayed his manipulation skills and destroyed the alliance within episodes, sending Cameron home by having him rebel against Jo, and Jo leaving in the following episode.
Once returning to the game, Alejandro immediately worked to make an alliance with people to secure his place. He easily attained Dakota, who did very little to help him, and he managed to break Nathaniel's spirits by convincing the whole cast that he's secretly bad, a rumor started by Heather. Nathaniel gave in, but was freed when the whole cast discovered the truth and eliminated Alejandro in Sweden Sour.
The biggest alliance yet. In The Ex-Files, Scott met privately with Beth and Lightning to form an alliance with their entire team, Team Pickle. Scott didn't trust his own team, which consisted of Alejandro, Nathaniel and Noah. Beth accepted, and the team made it all the way to the merge, where Nathaniel was accepted in and Izzy opted out in favor of Mel's alliance. After Dawn was eliminated due to Scott's strategy backfiring, he was kicked out of the alliance, whereas Courtney swept in and stole leadership from him. Courtney herself was almost kicked out of the alliance by Lightning once Nathaniel was eliminated, but the four, Scott, Beth, Lightning and Courtney, stuck it out till Planes, Trains, And Hot Air Mobiles, where the alliance was terminated completely so all could compete for themselves in the final sprint.
Mel had never formed any alliances before the point of the merge, instead taking out people one by one through sheer flirtation, but after being the sole member of Team Athens, Mel changed tactics once the merge hit. She convinced Izzy to ditch her own alliance to join her, and she did. They tried to get Lindsay to join, but she was eliminated before she could even partake in it. Mel abandoned Izzy too after Izzy became "Brainzilla", and thus terminated the alliance in N.A.S.A. Nots altogether.
Heather and Alejandro actually formed this alliance during the Total Drama World Tour Do Over special. These two discovered that they'd be in All Stars, and formed the alliance, speculating that they'd get far with each other's help. Eventually, there was tension and Alejandro broke from the alliance, causing Heather to try and bring it back together and improve it, before it fell out of her hands and dissipated altogether.
Jo made another alliance, her first one in Total Drama World Tour Do Over being humiliatingly unsuccessful. Jo formed an alliance with Duncan and Mel because they were a couple and she believed they'd stay loyal, which Mel did NOT end up being. Jo scrambled to fix the alliance, and eventually did, overpowering Heather's alliance and even making her join hers.
Mal's alliance wasn't as much an agreed 'alliance' as much as it was Mal overpowering Duncan and abusing him to do his bidding. Preying off of Duncan's bad memories of him and even physically harming him on occasion, Mal was able to successfully use Duncan until Harold managed to defeat Mal in a fistfight, which opened Duncan's eyes to how manipulative Mal is, not strong.