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Heavy Elements
Heavy elements such as carbon, nitrogen and oxygen are requirements for the support of life as we know it. Heavy elements include anything more complex than hydrogen and helium, the most basic elements on the periodic table. Life on Earth is based on carbon, so a habitable planet would need a similar chemical makeup to support us. Nitrogen and oxygen are the primary components of the air we breathe.
Distance from Parent Star
Much like Goldilocks' porridge, a planet's temperature must be "just right" to support human life. Scientists refer to this region within a star system as a "continuously habitable zone" or CHZ. A planet in the CHZ should have a temperature range that always allows for liquid water on some parts of its surface.
Age of Parent Star
To be habitable to humans, a planet obviously needs a parent star, but its age is important. A yellow dwarf star like Earth's sun has too little mass to explode in a nova and will eventually expand into a red giant, possibly growing to consume everything within Earth's orbit. For any planet to be considered habitable, its star will have to be young enough that this expansion won't occur for a long time.
Correct Atmospheric Mix
Oxygen is obviously a key requirement for human habitation on any world, but pure oxygen is not the answer. A pure oxygen atmosphere would be highly flammable, making fires of any sort a potentially lethal hazard. Earth's atmosphere is only 21 percent oxygen; it's made mostly of nitrogen -- 78 percent -- and the other 1 percent consists of various other gases. Because of the predominance of nitrogen in the atmosphere, fires have enough oxygen to get started but don't blaze out of control from a single match.
Regular Orbit
A habitable planet needs to remain a relatively constant distance from its parent star. If a planet has a highly elliptical orbit, like the dwarf planet Pluto, for example, it will only be warm enough to support life when it's closest to its parent star. Liquids on the surface will freeze as the planet nears the apogee of its eccentric orbit and make habitation very difficult, if not impossible.
Sufficient Size
For a planet to be habitable, it needs to be the right size as well. If it's too small, then it can't hold an atmosphere. Without an atmosphere, a planet also can't protect its inhabitants from ultraviolet radiation.
Proper Gravity
To facilitate human life, a planet needs a gravity similar to that of Earth. If it's too strong, then it will be difficult for humans to move around, and they will become easily exhausted when doing so. Too little gravity, as NASA's astronauts have shown, contributes to bone deterioration.
Gas Giants in Star System
Gas giants such as Jupiter and Saturn protect Earth by attracting comets and asteroids with their massive gravitational pulls. They act as a kind of bouncer for our star system and protect the inner planets from devastating impacts; sometimes, however, things get through to Earth, and scientists believe the dinosaurs were wiped out by one such impact. It's possible that having a gas giant in the outer part of a star system is necessary to protect the inner planets from constant bombardment and to allow life to develop and thrive.
Sizable Moon
Scientists don't know for certain whether a large moon is a requirement for life, but Earth's moon helps stabilize the planet's obliquity, which is the angle between Earth's orbital plane and its equator. If not for this stabilizing effect, planetary temperatures could vary wildly and make habitation difficult if not impossible.