Plagiarism is using someone else's ideas, words, images and passing them off as your own without acknowledgement.
Paraphrase: A restatement of a text or passage in other words.
It is extremely important to note that changing a few words from an original source does NOT qualify as paraphrasing. A paraphrase must make significant changes in the style and voice of the original while retaining the essential ideas. If you change the ideas, then you are not paraphrasing -- you are misrepresenting the ideas of the original, which could lead to serious trouble. [Source]