Not long ago, I wrote an article about the differences between formal and informal language with a special emphasis on the major difference between the two which is that one of meant for professional purpose
PURPOSES: A formal letter is typical meant for official purposes and as such must be written in formal language. This is the major differentiating factor between a formal letter and an informal letter because unlike the formal letter, an informal is by nature chummy; written to friends and relatives.
OBJECTIVES: In the corporate world as well as in government and any professional environment, there is inevitable need for constant communication. And letter writing is one of the means to ensure that this happens. Therefore, it can be said that the objective for writing formal letters is to ensure professional communication. This is almost similar to informal letter writing whose purpose is to ensure personal communication.
FORMAT: There are distinct formats for writing formal and informal letters. For one, the formal letter must have two addresses, starting with the writer’s address which must be on the top right corner of the page, followed by a date. And the second one (belonging to the recipient of the letter) should follow, on the left corner of the page just slightly adjacent the first address. But just in case the writer is writing on a Letterhead, then there would be no need to rewrite the writer’s address as it is already on the Letterhead. The purpose of the letter is then written in bold letters and centralized just before the salutation. The body of the letter should be concise and professional. No need talking about irrelevant stuff. It is business so keep it professional, simple and short; unless unavoidably necessary. The closing is equally just as important. Be polite, graceful even but not personal.
On the other hand, when writing an informal letter there is need [only] for one address. Letterheads are typically not used, and there is no for a topic. Greetings are acceptable but always in friendlier tones than in formal letters. In the body you get to talk about all kinds of personal stuff. And you can close by telling your recipient that you love them.
RECIPIENT: Formal letters are written to business partners, universities/employers and organizations etc. On the other hand, informal letters are written to friends, family and acquaintances.
TONE: Formal letters are passive in tone while informal letters are active.
SENTENCE STRUCTURE: When writing formal letter, the preferred sentence structure is for it to be long and complex. But for informal letters, it should be short and simple.