There are a number of ways to avoid committing plagiarism.
Summarizing is taking a big idea and distilling it down to a smaller explanation IN YOUR OWN WORDS. You still need a citation when you summarize something.
A summary of J.R.R. Tolkien's The Hobbit might be:
"A little man joins a wizard and a bunch of dwarfs on an adventure to recover a dragon's treasure, (Tolkien, 1937)."
Paraphrasing is taking a small section of your source and putting in YOUR OWN WORDS. You still need a citation when you paraphrase someone else's work. Paraphrasing will likely make up the majority of your research papers.
A sample of paraphrasing from The Hobbit might be:
Original Text:
“Now is the time for our esteemed Mr. Baggins, who has proved himself a good companion on our long road, and a hobbit of courage and resource far exceeding his size, and if I may say so possessed of good luck far exceeding his usual allowance—now is the time for him to perform the service for which he was included in our Company; now is the time for him to earn his Reward.”
Paraphrased Text:
"Thorin announced that it was time for Bilbo to fulfill his contract as burglar and scout out the dragon's lair, (Tolkien, 1937, p. 203)."
Quoting is using the author's exact text, either because the text perfectly states your point to help your argument, you can't come up with an alternative way to state the information, or because the text is so beautifully written, poetry, or song lyrics whose meaning may be lost if you paraphrase them. Place the quote inside quotation marks, and, of course, always include a citation.
A sample quotation from The Hobbit might be:
"Far over the Misty Mountains cold/To dungeons deep and caverns old/We must away, ere break of day/To seek the pale enchanted gold," (Tolkien, 1937, p. 27).