Some sources can be consulted for ideas or background, but cannot take the place of peer-reviewed journals. For example:
Websites - Avoid citing personal websites (even those of scientists) unless they are hosted by a scientific research institution.
Encyclopedias - Avoid citing encyclopedias unless they are science-specific and peer-reviewed, e.g. Encyclopedia of Life Sciences (eLS).
Magazines - Avoid citing general magazines (even science magazines like Discover or Science News).
Books - Avoid citing books unless they are written by and for scientists, and published by a scientific or university press. Books are usually secondary sources.