Human settlement and colonization of Mars would require complex life support measures and artificial environment. This is greatly due to the reduced air pressure and 0.1% oxygen in the atmosphere there. With 95% carbon dioxide in the air, the residents on Mars will not be able to go outside their 'homes' without pressure suits and bottled oxygen!
On Mars, it would be just about surviving in the environment there, rather than living. Whereas, we humans are made to live and enjoy and the atmosphere available on earth is just suited for that.
Hence the planned colonization of Mars would just mean to create earth-like atmosphere in controlled systems so that humans can survive.Mars would need a great amount of terraforming to allow humans to live unaided.
Mars can become a second home for humanity only if science is able to transform Mars so that it becomes a place where humans can live and breathe and enjoy unaided and not just create rooms and colonies with earth-like atmosphere where people would be spending their lives adjusting and surviving in an un-conducive environment.