I am reading a book called Classroom Instruction that Works.  It is primarily for teachers.  But I am learning things that help students (such as myself) as well.  

I just learned a great idea on summarizing. To summarize effectively students need to:

1. Delete some information

2. Substitute some information

3. Keep some information

This seems quite basic, even simple, but I didn't learn it until today.  Most of the time, I thought that a summary was just making something shorter.  If I had to summarize a story, I would just tell it in shorter words.  I never really tried to subsititue things unless it was changing a longer word for a shorter word.  

What summarizing really is, or what it should be for effective learning and communication, is getting to the heart of the problem.  The core.  Summarizing is finding what is essential and nothing more.