Nearly all application procedures require a one-page personal statement or essay. This important opportunity allows you to describe the special strengths you feel you can bring to the profession. It is your opportunity to persuade the admissions committee to select you. If you have anything in your background that needs to be explained, do it here. If you missed a year due to illness or family problems, or had a bad semester because of an unavoidably heavy work schedule, briefly discuss it here. Always emphasize the positive in any such discussions, stressing what you might have learned from the experience. Do not just make excuses.
Explain why you want to be a physician/PA/PT, etc. As you write, cover a few topics in some depth, and avoid simple declarative sentences about what you do or have done. These topics do not have to be related to your chosen profession, but should have some relevance to your professional potential. For example, a champion runner would let the committee know that she is more than a weekend jogger. She would emphasize that the same qualities that made her a champion (determination, a will to excel, and willingness to put in long, hard hours to achieve a goal) will contribute to her success as a student and practicing professional.
Anecdotal events are better than descriptive statements. If you are an empathetic, caring person, give an example of activities that illustrate this. If you have engaged in unique activities or travels, discuss them and how they might contribute to your training or work as a professional. Interesting topics make reading your essay more enjoyable for the admissions committee, and may elicit more positive feelings about the essay. Include only things you know and understand well, and would be comfortable discussing in person. An interviewer may very well pick something from your personal statement as a launching point for discussion.
When you apply through a central application service, the individual schools may send you a supplemental application form with yet another essay. Do not use the same essay! Some of these forms will also ask you specific questions, such as the reasons you wish to attend that particular school. Such questions are used to screen out students who are not genuinely interested, so answer them well. The final product of all your completed application materials must be flawless. Nothing less will do. Remember, you are competing with many other applicants for one position. If you were reading and evaluating this information, what initial impression would you receive from a form that had typographical or grammatical errors? Have several people proofread the essay for you.