A personal letter is written by a private individual and can be addressed to either another individual or a business.
Examples of a personal letter are the letter that a student would write a friend thanking him for a loan and summarizing his repayment intentions . . . a letter an interested buyer would write the owner of a car for sale . . . the letter one would write a hotel to confirm reservations . . . the letter asking the subscription department of a magazine to cancel a subscription . . . the letter alerting the billing department of a service provider to errors in an invoice . . . .
The personal letter deals solely with matters that concern an individual—not a group of individuals or a business.
A personal letter may also be written by an individual to make a personal announcement, such as a change in name or a change of address.
Regardless of the recipient and the reason for writing it, the personal letter represents you. Therefore, it must always say what you would say if you could be there in person, but without being overly emotional, casual, vindictive or careless.
A personal letter may be legally binding on both the person writing it and the individual/business to whom it is addressed.