Part 1 - Letterhead or Typed Heading
This has the company's name, address, and phone number.  It can also include the logo, and it the best practice is to place it in the center at the top.

Part 2 - Date
This is right justified and the month is spelled out.

Part 3 - Inside Address
This is the address of the person who is sending the letter.  This would include the name, position, and address of that person.

Part 4 - Salutation
For formal letters this starts off with Dear and includes their first and last name.

Part 5 - Body
This includes all of the text between the salutation and the closing.  Start of paragraphs should be indented, and a double space should be between each new paragraph.

Part 6 - Closing and Signature
This will include a complimentary closing, like "sincerely," along with your hand written signature and typed name.