Before: The Red Planet. After: The Pus-colored Planet. Believe it or not, it's actually an improvement.
Terraforming is the process of taking an uninhabitable planet and making it habitable. For a planet such as Mars, this means melting the polar icecaps to thicken the atmosphere, bringing in massive amounts of water and atmospheric gas chemicals (usually from cometary ices), and seeding the planet with genetically-engineered organisms (plants and algae to make oxygen, bacteria to break down chemicals in the soil and release nitrogen, and so on) to make the air breathable and the soil capable of supporting life. For a planet such as Venus, that would mean precipitating unwanted chemicals such as sulphuric acid and carbon monoxide out of the atmosphere with enormous atmosphere condensers, then introducing specialized organisms to break down other unwanted chemicals and introduce chemicals we do want into the air and soil. The process would normally take centuries, but by use of advanced nanobots and the supernatural abilities of Life Mages and Matter Mages, the process can be truncated to mere decades.