Terraforming is a (currently hypothetical) process that involves the formation of an atmosphere on a planet by raising the temperature, so that the world develops a biosphere which could support (carbon-based) life. The process increases oxygen level in the atmosphere as the planet becomes habitable for human life. For example, the terraforming process on a world that is covered in ice can be terraformed by increasing the level of greenhouse gases that convert the planet's climate to Earth-like. There are many different processes that transform a planet. Terraforming may take several centuries.
Goa'uld used devices for terraforming other worlds on which they could place their slaves. They used terraforming process on the planet Avnil. (RPG: "Fantastic Frontiers: Stargate Season One")
System Lord Sokar transformed Netu to resemble Hell. He filled the atmosphere with barely breathable toxins and blasted numerous holes into the moon's surface, exposing the molten core. (SG1: "Jolinar's Memories")
Lanteans had designed a device that could terraform an entire planet without having to take several centuries. (SGA: "Exogenesis")
The Gadmeer built a massive ship, which held enough resources to terraform a single planet into a world capable of sustaining their race. Such a world however would be inhospitable to human life. (SG1: "Scorched Earth")