The first part of this question has been thoroughly answered. However, the second part comes from an incorrect premise.
We can live on other planets. It's just that we need a very large amount of technology and supplies in order to do so. Currently, the cost of this support is too large for us to feasibly establish a colony on another world, and there are no locations we have yet discovered which do not require extensive life support just to get there, let alone to live there.
Terraforming is one procedure that might circumvent this. Terraforming is the act of altering a planet's ecosystem in order to be more like Earth. Some extreme examples imagine turning Mars or Venus into "second Earths" complete with oceans and atmospheres, but this is a bit of a stretch. Mars lacks important features, such as an active magnetic field, that would be necessary for habitability and cannot be engineered through current human technology.
However, terraforming could create localized regions in which human life is, at least, more generously supported than the rest of the planet. For example, living underground on Mars would be a lot easier than living on the surface, or trying to alter the entire planet.