There are many assumptions about life on other planets. Most scientists agree that the planet must meet the following conditions: stable orbit in the habitable zone, geologic activity, a stable weather pattern, and the right mass.
Now, if you are looking for a planet for humans to colonize, would the requirements be as rigid? To what extremes would the technology we currently posses, or in the process of developing, allow us to survive? How stable does the planet's orbit or tilt have to be? How small, or massive, can we handle? What conditions would be too much for us?