Formal :
Dear Dr. Brown, I am the chairman of the 2012 Metropolitan Medical Conference that is being held in Miami, Florida on July 5, 2012, and I would like to invite you to present your research on beta blockers. We would be delighted to listen to you discuss your work, and I hope you will also agree to run a short question-and-answer session after the presentation. Additionally, Metropolitan Medical would be pleased to cover your travel and lodging expenses while you visit the conference, in addition to providing you with a per diem budget.   Please reply with your answer as soon as you are able, so that we may begin to arrange your travel. I encourage you to contact me with any questions or concerns.
Kind regards,
Dr. Michael Smith 

informal :

Dear Grandma, How are you? I was really happy to see you and Grandpa at my graduation ceremony, and I hope you’re recovering from your cold. Do you think it would be okay if I stopped by to visit you on Saturday? I’d really like to show you my new puppy. I think you’ll adore her. Her name is Sassy, and I got her last week. And she’s your favorite breed: a golden lab, just like Satchmo! I’m excited to come see you soon. Let me know if Saturday will work for you.