What is terra forming in space?

Terra Forming is an idea discussed amongst scientists and Science Fiction writers alike. In the far future, as man travels to other uninhabited planets, they could modify a planet's harsh environment to become a planet more like earth. Mankind could inhabit, live, and thrive on this modified planet just like on earth.

Modification might be done with seeding, cloud seeding, moving a planet closer to or farther from its star, etc