Summaries are meant to be shortened versions of their referenced materials, encapsulated in a short text of some sort. The key to a great summary is to boil down the meaning, goals, or overall idea of its referenced material. Summaries are usually written as text and can be as short as one sentence, or as long as several pages in length. The length depends on the expectations based on other summaries. For example, the summary for a movie is typically about three to four sentences in length, so a one-page summary would seem out of place. Summaries should aim to contain the central ideas in the reference material, not just a taste.
For practice, let's consider writing summaries for various materials.
The Harry Potter series: "Harry Potter is a seemingly normal teenage orphan who discovers he is the son of famous and powerful wizards. He enrolls in Hogwarts, a school for young magicians, and embarks on a journey of self-discovery. Along the way, he meets friends and foes, increases his ability to do magic, and tries to save the world from the evil person who killed his parents."
A well-written summary will include a beginning, middle, and an end. Again, you're not trying to entice people to read the story or watch the movie or read the paper, you are trying to give a complete overview of every idea contained therein.
A summary of climate change: "Climate change is the idea that the earth is warming and the icecaps are melting, mainly due to human activity with fossil fuels, which causes CO2 emissions to keep the sun's heat trapped in the Earth's atmosphere. Many fear that this change in temperature will cause sea levels to rise, and a massive change in climate around the world."
Again, we are encapsulating the main idea in a very short amount of space. Obviously there is more to the story than what is written in the summary, but the summary provides a good overview.