What is a Statement of Purpose?

As the name signifies, the Statement of Purpose is your personal statement about who you are, what has influenced your career path so far, your professional interests and where you plan to go from here. It need not be a bald statement of facts; several successful SoPs address these questions through anecdotes, stories or by describing their hero. But whether your SoP is subtle or to the point, it must be well written to be successful. 
This is because the SoP is the only part of your application packet over which you have full control. Your academic and extra-curricular records are in the past. Most people only take one or two shots at the GMAT, GRE or TOEFL, and these scores could be adversely affected by conditions on the test day.  It is important to choose recommendation letter writers carefully, but while you hope they give you the best possible recommendation, this is not within your control. 
The SoP is your chance to talk directly to the admissions committee. To make yourself stand out from  among a multitude of similarly qualified candidates. To convince the committee that you have the spark, the thirst for knowledge that could add value to your class.
Most of us work hard for the standard tests – the GRE, GMAT, TOEFL and others. We attend classes or peruse study aids. We give practice tests and do everything within our power to aim for the highest possible score. Because we know that these test scores, while not a perfect tool, are crucial to our chances of gaining admission and even a scholarship or assistantship.
The SoP or essay, on the other hand, is put off till the last possible moment. It scares us – we look at those oh-so-perfect essay examples in the admissions guidebooks and wonder how we can ever write so well. Or wonder what shining instance we can pick out of our normal, average lives to show that we are unique and remarkable. Or how to pick our way through the minefield of endless Do's and Don'ts. Or, after overcoming all these obstacles, we falter at the seemingly endless revisions, wondering if this latest draft is good enough (If I read that essay once more, I'll scream!).
Finally we write something, because time’s a-pressing and we have to meet the application deadline. We do our best, juggling the writing process with the last-minute paraphernalia of applying – checking forms for errors and completeness, collating the application packets, making sure transcripts, recommendations, work samples and resumes go in their right envelopes, worrying about transit times.  We feel thankful when the essay is over, do a quick scan for obvious mistakes, and send it on its way. 
If you do it this way, you are practically throwing away your chances of admission (see the next section, What do Schools look for in a Statement of Purpose?). A good SoP will certainly improve your chances of getting admission to the school of your choice, and even compensate for weaker portions of your application such as less-than-perfect grades. A bad SoP, on the other hand, has the potential to drag down an otherwise strong application.
If you plan correctly, you can give yourself enough time to submit a well-written, thoughtful, polished essay that will boost your chances for admission. Equally important, this is a great opportunity to look inside yourself and be rewarded by a better understanding of who you are.
Writing a reasonably good Statement of Purpose is not an impossible task. It requires care, attention and patience. And enough time for you to be able to write several drafts, show them to people and polish the essay till you get a version you are happy with. 
Done right, this will even turn out to be an enjoyable process. And you will be the richer for it. Read on.