A concise summary is one in which the author's main ideas have been captured in a few well-chosen words, and supporting details have been characterized in a general way or reduced to a few representative details. A good strategy for capturing the key ideas is to go through the entire text and write the main point of each paragraph in the margin or on note paper. Then, go over your notes and ask yourself how each paragraph fits into the author's overall project: Why is it there? How does the paragraph advance the author's argument or goal? This step of shifting from what is there, to why it is there is absolutely essential in a good summary, for it will help you to characterize the work as a whole, showing how the parts fit together.
Though the goal in summary writing is to be concise and eliminate unecessary supporting details, it is also important not to misrepresent a work by leaving out important sections of an author's text. If an author spends a substantial amount of time and space on a topic, a summary should include at least a brief explanation of why that section is there. A complete summary is one that does not skip over key points.
Lesson: Writing Summaries - Santa Rosa Junior College
Before you write the summary, consider why your audience (professor, boss, client) wants to read it. Why shouldn't the reader just read the original? Summaries benefit the reader because they offer a concise, general version of the original information. For a busy reader, summaries provide quick overviews of material. Summaries also show readers that you have understood the general point of a text, and in this way, teachers can test your knowledge. The process of summarizing someone else's material enables you to better understand that material. Finally, summaries allow you to introduce knowledge within a research context: you can summarize someone's argument in order to analyze or critique it.