Although this makes it redundant to put "Please RSVP" at the bottom of an invitation, redundancy in the name of politeness can be forgiven.
RSVP is an acronym, which is a word made up of initial letters, as in scuba (self-contained underwater breathing apparatus) and laser (light amplification by stimulated emission of radiation).
By the strictest definition, RSVP is an initialism, because the letters are pronounced as letters, as in FBI, not as a word, as in scuba and laser.
Some dictionaries don't draw this distinction, but plainly these are two different kinds of words, and in practical usage the biggest distinction is that initialisms are written as upper case letters (as they are here), while acronyms, once we get used to them, are often written in lower case. (This is why it's funny when Dr. Evil, having been out of circulation in the decades that "laser" became a part of our everyday vocabulary, uses those quotation marks in the air around "laser beam" to indicate that he is using a highly technical acronym.)
The discussion of RSVP brings up two other fossils in our language:
- Those accents, one over the é and one over the î. What are they for? An amateur linguistic explanation is that they stand for "s"es that were long ago dropped from the French language. This goes back to 1066, when the Normans, a bunch of Vikings who had settled in Normandy and taken up French , sailed over and conquered England. They sounded their "s"es, so the French words in our language have those "s"es in them, like respond, please, forest (forêt), and many others. Back in France, in the centuries after 1066, the French took to dropping their "s"es--perhaps the way Bostonians drop their "r"s (as in "Pahk the cah in Hahvahd Yahd").
- Why do we call capital letters "upper case" and small letters "lower case"? This is a printing term, from the early days of movable type. The printer kept the big letters in the upper case, and the small letters, which he used much more often, in the lower case, where he didn't have to reach as far. (This is akin to the bartending term "top shelf" for the most expensive liquors. To this day, bartenders keep the priciest brands on the top shelf, where they have to reach less often than for the cheaper brands.)