A habitat is an environment in which an organism, or group of organisms live in symbiosis.
The environmental conditions on Mars do not allow human beings to live without technical equipment, not even for a minute. While people on Earth can live from the nature's air, water, etc. this is not an option on Mars. So, the people's habitat is an artificial environment inside of buildings, rovers and space suits.
Ideas of terraforming intend to make the whole surface of Mars habitable.