Paraphrases—rewordings of text—need to be cited. Paraphrasing without providing a citation is plagiarism. Even paraphrases with citations can be instances of plagiarism if they are so similar to the original that the paraphraser claims credit for the original author's language.
A paraphrase that avoids plagiarism:
Wines drunk at Greek tables did not always come from Greece itself. The wine snobbery of the time extolled the merits of wines from the slopes of Mount Lebanon, from Palestine, Egypt and Magna Graecia-Greater Greece, i.e., southern Italy. The ten litres a day drunk by the famous wrestler Milo of Croton was a wine famous in Calabria, where Milo lived: this wine, Ciro, is still made.
from Maguelone Toussaint-Samat's A History of Food (Cambridge: Blackwell, 1992. 263).
Wines drunk by Greeks were not always made in Greece itself. The wine snobs of that period celebrated wines from Mount Lebanon, Palestine, and Egypt. The famous wrestler Milo of Croton, who consumed ten liters of wine a day, drank wine made in Calabria outside of Greece; this wine, Ciro, is still made.
This paraphrase plagiarizes in two ways:
1. By having no citation, the paraphrase misleads readers into believing that the ideas, facts and sense of the passage are a result of the author's own research and knowledge.
2. The language of the paraphrase is too similar to the original. Even if the author had provided a citation, some instructors would consider this plagiarism.
Although Greeks were picky about their wine, they enjoyed wine from outside Greece. Upstanding Greeks enjoyed wine from many of Greece's local trading partners—including Palestine, Egypt and southern Italy. One story tells of the famous wrestler Milo of Croton, who consumed ten liters of foreign wine daily (Toussaint-Samat 263).
This paraphrase cites the original and rephrases its words to create an original construction.
Up, up, up, groping through clouds for what seemed like an eternity....No amount of practice could have prepared them for what they encountered. B-24s, glittering like mica, were popping up out of the clouds all over the sky.
from Thomas Childers. Wings of morning: the story of the last American bomber shot down over Germany in World War II, Reading, Mass.: Addison-Wesley (1990), 83.
Up, up, up he went, until he got above the clouds. No amount of practice could have prepared the pilot and crew for what they encountered-B-24s, glittering like mica, were popping up out of the clouds over here, over there, everywhere.
This comes from The Wild Blue: The Men and Boys Who Flew the B-24s over Germany by Stephen E. Ambrose. Ambrose cites but does not quote Childers' original work despite using its imagery and language. Ambrose should have either used Childers' passage as a direct quotation or modified his own passage so that it consisted of his own language.
Despite their training, the pilot and crew's experience was surreal and surprising, seeing for the first time "B-24s, glittering like mica, ... popping up out of the clouds all over the sky" (Ambrose 83).