If you are thinking of applying for a PhD program in the UK, you will have to write a personal statement to support your application. The point of a personal statement is to tell the admissions tutors a little about yourself. Grades can only say so much about a person. The personal statement is an opportunity for you to demonstrate why they should pick you over other applicants. An effective personal statement demonstrates a strong interest in the chosen subject, while highlighting the applicant's knowledge and experience.
Introducing yourself. Explain which PhD program you are applying for and the specific subject of your research.
Explain your interest in the subject. What inspired you to pursue this particular PhD? Because it takes years to complete a PhD, universities want to see that you are passionate about the subject and committed to finishing what you start.
Explain why you want to go to this particular institution. Do some research about the specific department so it doesn't seem like you chose it at random.
Discuss what skills and experience you have that will help you do well in this PhD program. If you're already working in the field, mention any relevant accomplishments.
Explain what you want to do once you've finished the PhD. Do you want to go into academia? Would you like to teach or write books? Universities like to see that you have thought this through.
Be specific. Generalities make it look like you sent the same letter to five other universities. Make sure that every paragraph is tailored for this specific program at this specific school.
Limit yourself to 300-400 words. The admissions tutor will have many applications to read and not much time to devote to each.
Proofread your personal statement carefully. If you have a friend who has done a PhD in the same field, or is in the process of doing one, ask him to look it over for you.
A personal statement is not the same thing as a research proposal. The latter will be much more detailed. If you have to go in for an interview, be prepared to discuss your personal statement.