plagiarism - from Latin plagiarus (kidnapper); taking else's info and presenting it as one's own  

• type of intellectual theft because it's taking someone else's ideas without their permission
• type of fraud because it's also passing off someone else's ideas as your own

unintentional plagiarism  

• using author's exact wording without crediting them
• forgetting to credit authors
• copying the syntax (sentence structure) of another author

forms of plagiarizing - the following would be okay if you gave credit to the original author  

• copying and pasting from the internet
• using specific facts and statistics
• repeating someone else's choice of wording
• using someone else's sentence structure
• replacing certain words in a sentence with synonyms and then calling it your own
• using four or more words from the original source
• using a term invented by another author
• using someone else's opinions and way of thinking on a certain topic

Occasions when documentation of sources is not needed  

• using a well-known quote
• using a well-known proverb
• using a well-known fact
• using information available in a lot of sources
• information that you already know