When you summarize from any given point of reference, you mention main concepts from your source. Paraphrasing the same source is a slightly more involved process. A proper paraphrase matches almost sentence to sentence. Therefore, a summary will end up considerably shorter than the original source while a paraphrase of the same work will be about the same length.
When summarizing, you have somewhat more leeway, as you are covering only main concepts and the most essential elements of the original written piece. You must be extra careful when paraphrasing, however, because the verbiage in your paraphrased work must be noticeably different from the original passage. Otherwise, you are at risk of committing plagiarism. Merely changing a few words throughout the passage does not constitute a proper paraphrase.
As previously mentioned, you have more liberties when it comes to summarizing than you do with paraphrasing. About the only liberty you have with the latter is the opportunity to change the tone of the original written work. For example, if the original was written in casual prose, you can compose your paraphrase using a formal tone -- and vice versa. In addition, with any paraphrase you attempt, you must cite the original work from which you paraphrased.
In summarizing, you are presenting a general outline of the referenced source. The purpose of paraphrasing, on the other hand, is to give a more detailed presentation of the author's ideas and arguments -- without directly quoting from the original passage.