It was a blue liquid, created by various combinations of chlorofluorocarbons, vinyl acetates, copolymers, maleic anhydride, and ethyl alcohol. Anti-plastic interacted with the molecular structure of plastic and caused it to disintegrate. When put in water, it bonded with the polar hydrocarbon molecules, adhering to them and replicating itself throughout the water. (PROSE: Autonomy)
The Ninth Doctor carried the liquid around in a vial in his pocket. He had originally planned to use the anti-plastic as a threat and a bargaining chip to make the Consciousness leave Earth, which the Consciousness refused to do. Discovering the Doctor's TARDIS, the Consciousness had an Auton take the Doctor's anti-plastic from him.
When Rose Tyler rescued the Doctor, she knocked over the Auton holding the vial, who fell into the Consciousness. When it did so, the substance reacted explosively with the Nestene Consciousness, which led a halt of the attempted Auton invasion. (TV: Rose)
A vial of anti-plastic, implied by Kate Stewart to have been provided by the Doctor, was possessed by UNIT and was secretly kept in Black Archive 5 which had been decomissioned in 2006. Recovering the vial from the archive, Kate brought it with her as a plan B for her confrontation with the Nestene Consciousness during its 2010s invasion. It was used to severely wound the Nestene Consciousness, but failed to defeat it. (AUDIO: Armageddon)