You are required to identify for your reader which ideas, facts, theories, concepts, etc., are yours and which are derived from the research and thoughts of others. Whether you summarize, paraphrase, or use direct quotes, if it's not your original idea, the source must be acknowledged. The only exception to this rule is information that is considered to be common knowledge [e.g., George Washington was the first president of the United States]. If you are in doubt about whether a fact is common knowledge or not, protect yourself from any allegations of plagiarism and cite it, or ask your professor for clarification.
Listed below are particularly well-done and comprehensive websites that provide specific examples of how to cite sources under different style guidelines.
This guide provides good information on the act of citation analysis, whereby you count the number of times a published work is cited by other works in order to measure the impact of a publication or author.
Measuring Your Impact: Impact Factor, Citation Analysis, and other Metrics: Citation Analysis [Sandy De Groote, University of Illinois, Chicago]
Type in your information and have a citation compiled for you. Note that these are not foolproof systems so it is important that you verify that your citation is correct and check your spelling, capitalization, etc. However, they can be useful in creating basic types of citations, particularly for online sources.