There is no absolute definition of life on Earth, so it is really difficult to define life on other planets. By "life" we mean simple bacterial life not ET as bacterial life is expected to be far more abundant and easier to find once we know where to look and what to look for.
Conditions for Life
Based on what we do know about life on Earth, simple life requires a few basics in order to appear.
Basic life requires that enough time has passed for stars, planets and a suitable environment to form, including heat from within the core - but not too much, geological processes and an atmosphere to form. Also, enough time must have passed for Darwinian evolution to create simple living organisms from the organic molecules.
2. Habitable Zone
The planet needs to be within the habitable zone of the solar system, a location where liquid water can exist. The position within the habitable zone and the planet's mass gives a good indication of its surface conditions. An Earth-mass like planet would not have enough mass to be a gas giant, and it would be too hot to be an icy body so chances are it has a rocky surface. The habitable zone (or Goldilocks zone) is explained later in this article.
3. Raw Materials
There must be enough raw materials such as Carbon, Hydrogen, Iron and Oxygen which have formed inside massive stars and been seeded to plants. These can then combine to form organic molecules or a "primordial soup" where the building blocks of life accumulate.
Finally, a catalyst is required for the organic molecules to combine and form into organisms.
The Habitable Zone
The habitable zone refers to two areas - one within a solar system and the other within a galaxy. The circumstellar habitable zone is a distance range from a star where a planet can theoretically support life and is defined as "the range of distances from a star for which liquid water can exist on a planetary surface." This habitable zone is situated such that the temperature of the surface is not too hot, not too cold but just right (hence the Goldilocks name).
The range of distance will be affected by the energy output of the star, with hot supergiants having a habitable zone much further out than a red giant.
The Habitable Zone can also refer to the galactic habitable zone, which is the area within a galaxy that has the correct balance of heavy elements and radiation. In general, the outermost parts of a galaxy do not have enough heavy elements, whereas the central core contains too much radiation (x-ray, gamma and so on).