As it sounds, the habitable zone is a region capable of supporting life in a Solar System. Two different regions must meet the right conditions in order for it to be possible for life to exist. The circumstellar habitable zone (CHZ) is limited to a Solar System while the galactic habitable zone (GZH) is a hypothesized spherical band that may be a likely place for terrestrial life to develop in a galaxy. This hypothesis is still in the very basic stages and very little research has been done on it so far. The GZH is essentially the circumstellar habitable zone on a larger scale.
The circumstellar habitable zone, also known as the ecosphere, is thought to exist around all stars, not only the Sun. If a planet is outside of this habitable zone than it is believed to be incapable of sustaining life because outside of this band it is impossible for liquid water to exist. The most important factor in the possibility of life is the presence of liquid water because its role as a solvent in biochemical reactions is essential.
Earth is in the middle of our solar system’s habitable zone with Mars and Venus on either side. The exact limits of our Sun’s CHZ is unclear, which is one reason why scientists are so interested in finding life on Mars. If astronomers do fail to find life on Mars though, that may be due to atmospheric differences between the countries rather than the boundaries of the CHZ. Venus’ proximity to the Sun has caused it to develop an atmosphere with extreme temperatures that would be unsuitable for any kind of Earth-like life. The size and location of CHZs of other stars depend on various factors, such as the size, brightness, and temperature of the star and other planetary factors.
The galactic habitable zone is a hypothesis supported by various astronomers, though the idea has been met with some skepticism. The general idea is that the center of the universe acts comparably to the way that the Sun acts in the CHZ. The favorable spot would have to be close enough to the center of the universe so that it could gather enough heavy materials from the center to form planets and complex life, but far enough away to have some protection from dangerous radiation, which would damage any carbon-based life forms. Astronomers point out that one of the flaws of the habitable zone theory is that it only takes into consideration carbon-based life, which means that the possibility of some other form of life unknown on Earth could exist outside of the habitable zone.
Universe Today has more articles on this subject including using gravity to find planets in the habitable zone and other habitable earths.
Astronomy Cast also has an episode that you will want to check out: search for water on Mars.