Honorific:  a word conveying esteem or respect when referring to or addressing a person
     The most common honorifics are Ms., Mrs., Mr., and Miss, but there are many others, including Dr., Captain, Coach, Reverend.  Honorifics Ms., Mrs., and Mr. are typically followed by a period, but not always, especially in British publications. Miss is not, as it is not an abbreviation for a word at all.  Actually, Ms. really is not either, so the period is optional.  Confused yet?  Here's a snapshot:
>  Miss
>  Ms or Ms.
>  Mrs.
>  Mr. 
     There's more!  Miss is typically reserved for a young lady of the age 18 or younger, however it is acceptable to use with a Bride to be.
>  Miss Lisa Simpson (under age 18)
>  Miss Lisa Simpson (age 21, soon to be married)

     Ms. should be used as the slightly formal honorific for all women over the age of 18, regardless of marital status.  Use Ms. when you are writing notecards on both the note salutation (with last name only) and the address on the envelope (with first and last name).
>  Dear Ms. Simpson,  (salutation)
>  Ms. Marge Simpson  (on envelope)
    123 Main Street

   You may also use Mrs. along with the last name as long as you do not use the woman's first name.  Mrs. should never be separated from the man's name.  Ms. is always acceptable for married women, so feel confident in defaulting to Ms. if you are not sure of marital status.  I use it as a slightly formal sign of respect, although be cognizant that very formal acquaintances, especially society women of the south, will appreciate the formality of only addressing them as Mrs. followed by their husband's full name.
>  Mrs. Butler
>  Mrs. Rhett Butler
>  But never Mrs. Scarlet Butler, only Ms. Scarlet Butler

     Now practice your skills by writing a note to someone lovely who deserves a dash of your kindness today!

                                                                                                                                  - *hk*