When in doubt, cite your source!
Identify words that you copy directly from a source by placing "quotation marks" around them, typing them in a different color, or highlighting them. (Do this immediately, as you are making your notes. Don't expect to remember, days or weeks later, what phrases you copied directly.) Make sure to indicate the exact beginning and end of the quoted passage. Copy the wording, punctuation and spelling exactly as it appears in the original.
Plagiarize: to steal and pass off (the ideas or words of another) as one's own : use (another's production) without crediting the source (http://www.merriam-webster.com/dictionary/plagiarize)
There is a difference between fact and opinion. Understanding this difference can help you avoid unintentionally plagiarizing someone else's work. Generally, you do not need to cite the sources of generally accepted facts, but you DO need to give credit for original ideas and opinions that are not your own.
For example:
FACT: John Wilkes Booth assassinated Abraham Lincoln.
This is a factual statement which does not need credited to anyone. However,
OPINION: "John wilkes Booth assassinated Abraham Lincoln because he hoped that the assassination would create sufficient chaos within the Union that the Confederate government could reorganize and continue the war if one Confederate army remained in the field or, that failing, to avenge the South's defeat."
Here an author provides his rationale, or reasoning, for why he thinks Booth assassinated Lincoln. Generally, this idea could be up for debate, proved correct, or eventually disproved altogether. Since the author is providing us with his opinion, or his original idea about this issue, you would need to provide your readers with a citation for the source of this information.
NOTE: Other types of factual material, such as statistics, charts and graphs, and other information compiled through research, DO need a citation.