The personal statement is also a great place to explain more serious problems, like less_than_ideal grades or test scores. Don't be afraid to write something like, "I don't test well" in the statement. As long as you can provide a good reason for you app's problems, it's okay to address them directly. You won't be able to hide it from admissions officers anyway, so providing a reason for problems is always preferable to ignoring them.
There are schools that require an applicant to submit a Statement of Purpose instead of a PS, the thrust of which is to present an applicant's goals in terms of what major to pursue, what research direction to take, and other specific study and career plans, if any. Therefore, in this case, you are required to submit a similar essay but with greater emphasis on your specific study and career plans. Most applicants are confused with the difference between a PS and a Statement of Purpose. This is not to say, however, that a PS cannot be a Statement of Purpose and vice versa, because as you will discover in the next section, these two can actually overlap depending on the outline you have created.