A biographical source.
An encyclopedia of literature.
An encyclopedia of earth science.
They are good for providing background information, but are not usually cited in your paper.
They provide the most up-to-date information about a topic.
They are primary sources.
Be as broad as possible so that you have plenty to write about.
Be as narrow as possible so that your reader is not overwhelmed with details.
Be stated clearly enough that you can use it as a guide to selecting or rejecting possible research sources.