Anyone who makes the game experience negative intentionally, through pinging, blaming, affecting the morale of the team by being overly negative, or purposely assisting the enemy team. Choosing a build that you know does not work at all is trolling too, not at all the same as trying out new builds, but I'm talking about stuff like building full AP on Blitz when you are the support, stuff that would clearly not work.
They can assist the enemy team many ways: entirely ignoring their lane, AFK'ing, dying on purpose, or using CC to help the enemy kill you (like Tahm spitting you into an enemy when you only have 10 hp left, or blitz pulling an enemy to you at the same time, Bard helping an enemy escape by freezing you)